Mendel's experiments: dominance and segregation
A child may have the nose of one parent and the hair of the other; sometimes a trait skips a generation. A monk growing peas explained why.
Gregor Mendel crossed garden pea plants for seven years (1856–1863), using 14 true-breeding varieties that formed seven pairs of contrasting traits. He proposed that "factors" (now genes) occur in pairs called alleles. In a monohybrid cross the F₁ shows only one parent's trait (dominant), and in F₂ the two traits appear in the ratio 3 : 1.
In the figure : The dwarf trait disappears in F₁ and returns in a fourth of F₂.
In the figure : Genotypic ratio 1 : 2 : 1; the phenotypic ratio is 3 : 1.
Rule 1: Law of dominance — Characters are controlled by discrete units called factors that occur in pairs. In a heterozygote one factor (dominant) expresses itself and hides the other (recessive).
- Easy : Tt plant is tall because T is dominant.
- Medium : The recessive trait appears only when the genotype is homozygous recessive (tt).
- Exam-type : Q: What is the phenotypic ratio of a Tt × Tt cross? A: 3 tall : 1 dwarf.
Rule 2: Law of segregation (purity of gametes) — The two alleles of a pair separate during gamete formation so that each gamete gets only one allele. The alleles do not blend in the heterozygote; they reappear unchanged in the next generation.
- Easy : A Tt plant makes T and t gametes in equal numbers.
- Medium : Segregation of alleles explains why the recessive trait reappears in F₂.
- Exam-type : Q: Why is segregation called the law of purity of gametes? A: Each gamete carries only one allele of a pair, never both.
Genotypic ratio 1:2:1, phenotypic ratio 3:1 for the F₂ of a monohybrid cross. Mendel published in 1865; the work was rediscovered in 1900 by de Vries, Correns and von Tschermak.
Saying that the F₁ is a blend of the parents. In Mendel's peas the F₁ looked like one parent only.
Memory clue: 1-2-1 / 3-1 — Genotype 1-2-1, phenotype 3-1.
Spot the difference
| Point | Genotype | Phenotype |
|---|---|---|
| Meaning | Genetic make-up | Observable trait |
| F₂ ratio (monohybrid) | 1 TT : 2 Tt : 1 tt | 3 tall : 1 dwarf |
| Example | Tt | Tall |
Practice questions
- 1Who is called the father of genetics?
- 2What is an allele?
- 3What is the phenotypic ratio in the F₂ of a monohybrid cross?
- 4Name the plant used by Mendel.
- 5What do you call an organism having two identical alleles?
- 6What is the genotype of a pure dwarf pea plant?
- 7Why does a recessive trait reappear in F₂ although it was hidden in F₁?
- 8A Tt plant is selfed. What fraction of the tall progeny is homozygous?
Show answers (try them yourself first)
- 1Gregor Mendel.
- 2One of the alternative forms of a gene.
- 33 : 1.
- 4The garden pea (Pisum sativum).
- 5Homozygous.
- 6tt.
- 7Alleles segregate, so some F₂ plants get two recessive alleles (tt).
- 8One third (1 TT out of 3 tall plants).
Test cross and deviations from dominance
A tall pea plant may be TT or Tt. A simple breeding trick tells which, and some flowers (like pink snapdragon) refuse to follow the dominant–recessive pattern.
A test cross is the cross of an organism of unknown dominant genotype with a homozygous recessive plant; all dominant progeny means it is homozygous, a 1 : 1 ratio means heterozygous. In incomplete dominance the F₁ is intermediate (red × white snapdragon gives pink). In co-dominance both alleles express in the F₁ (human AB blood group).
In the figure : In incomplete dominance the phenotypic ratio equals the genotypic ratio, 1 : 2 : 1.
In the figure : Iᴬ and Iᴮ are co-dominant; both are dominant over i.
Rule 1: Test cross — Cross the unknown organism with a recessive homozygote (tt). If the offspring are all tall the parent was TT; if they are half tall and half dwarf, the parent was Tt.
- Easy : TT × tt gives all Tt.
- Medium : Tt × tt gives 1 Tt : 1 tt.
- Exam-type : Q: What is the use of a test cross? A: To find whether a dominant-trait organism is homozygous or heterozygous.
Rule 2: Multiple alleles and co-dominance — When a gene has more than two alleles in the population it shows multiple allelism. The ABO gene has three alleles Iᴬ, Iᴮ, i, but an individual has only two. Iᴬ makes A sugar, Iᴮ makes B sugar, i makes none. A person with Iᴬ and Iᴮ has both sugars on red blood cells (co-dominance).
- Easy : Group O has genotype ii.
- Medium : Iᴬi gives blood group A.
- Exam-type : Q: How many blood groups and genotypes are possible? A: Four phenotypes (A, B, AB, O) and six genotypes.
A diploid person has only two alleles of the ABO gene, even though the population has three. Incomplete dominance: 1:2:1 phenotype and genotype; co-dominance: both alleles show.
Writing that incomplete dominance is also co-dominance. In co-dominance both traits show separately; in incomplete dominance the trait is a mix.
Memory clue: Pink → In-between — Incomplete dominance = intermediate; co-dominance = both visible (AB).
Spot the difference
| Point | Incomplete dominance | Co-dominance |
|---|---|---|
| F₁ result | Intermediate trait | Both traits shown |
| Example | Pink snapdragon | AB blood group |
| F₂ ratio | 1 : 2 : 1 | 1 : 2 : 1 (genotype) |
Practice questions
- 1What is a test cross?
- 2What is the F₁ colour of red × white snapdragon?
- 3Name the alleles of the ABO blood group gene.
- 4What is the genotype of blood group O?
- 5What do you call a gene with more than two alleles?
- 6What are the genotypes for blood group A?
- 7A tall plant crossed with a dwarf gives 50 per cent tall and 50 per cent dwarf. What is the genotype of the tall plant?
- 8A father has group A (Iᴬi) and a mother has group B (Iᴮi). List the possible blood groups of their children.
Show answers (try them yourself first)
- 1Cross of an organism with a homozygous recessive to find its genotype.
- 2Pink.
- 3Iᴬ, Iᴮ and i.
- 4ii.
- 5Multiple allelism.
- 6IᴬIᴬ or Iᴬi.
- 7Tt (heterozygous).
- 8A, B, AB and O.
Dihybrid cross and independent assortment
Round or wrinkled, yellow or green: two traits tossed like two separate coins. Each coin falls independently of the other.
In a dihybrid cross two traits are followed together. Mendel crossed round yellow (RRYY) with wrinkled green (rryy) peas; F₁ was all round yellow (RrYy) and F₂ had four phenotypes in the ratio 9 : 3 : 3 : 1. The law of independent assortment says that when two pairs of traits are considered, the alleles of one pair separate independently of the alleles of the other pair.
In the figure : The 9 : 3 : 3 : 1 ratio comes from four types of gametes combining in 16 ways.
Rule 1: Gametes of RrYy — Four types of gametes are formed in equal numbers: RY, Ry, rY and ry. A 4 × 4 Punnett square gives 16 combinations.
- Easy : RrYy makes RY, Ry, rY, ry gametes.
- Medium : Each gamete type is 25 per cent.
- Exam-type : Q: How many types of gametes does RrYy make? A: Four.
Rule 2: Reading the ratio — Every single trait still follows 3 : 1: round : wrinkled = 12 : 4 and yellow : green = 12 : 4. The new combinations (round green and wrinkled yellow) are the recombinants.
- Easy : Round green is a new combination.
- Medium : Out of 16, nine are round yellow.
- Exam-type : Q: What fraction of F₂ is wrinkled green? A: 1/16.
Dihybrid F₂: 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green. Independent assortment works only for genes on different chromosomes (or far apart on the same chromosome).
Writing 9 : 3 : 3 : 1 as a genotypic ratio. It is the phenotypic ratio; the genotypic ratio has 9 classes.
Memory clue: 9-3-3-1 — Both dominant 9, one dominant each 3 and 3, both recessive 1.
Spot the difference
| Point | Monohybrid | Dihybrid |
|---|---|---|
| Traits studied | One | Two |
| F₂ phenotypic ratio | 3 : 1 | 9 : 3 : 3 : 1 |
| Punnett square | 2 × 2 | 4 × 4 |
Practice questions
- 1What is a dihybrid cross?
- 2What is the phenotypic ratio of a dihybrid F₂?
- 3How many types of gametes does RrYy produce?
- 4What is the genotype of the F₁ in RRYY × rryy?
- 5How many boxes does the Punnett square of the F₂ have?
- 6Which two phenotypes are the new combinations in the F₂?
- 7Why is Mendel's second law not true for all gene pairs?
- 8In the F₂ of RrYy × RrYy, what is the proportion of round seeds?
Show answers (try them yourself first)
- 1A cross between parents that differ in two pairs of traits.
- 29 : 3 : 3 : 1.
- 3Four.
- 4RrYy.
- 5Sixteen.
- 6Round green and wrinkled yellow.
- 7Genes located close together on the same chromosome are linked and do not assort independently.
- 812/16 or 3/4.
Chromosomal theory, linkage and recombination
Genes are like beads on a string; beads next to each other are carried together when the string is passed on.
Sutton and Boveri noticed that chromosomes behave like Mendel's factors (they pair and separate in meiosis), and so proposed the chromosomal theory of inheritance. T.H. Morgan worked on the fruit fly Drosophila and found that genes on the same chromosome are inherited together (linkage); new combinations arise by recombination (crossing over). Alfred Sturtevant used the frequency of recombination to map genes.
In the figure : The farther apart two genes are on a chromosome, the more often they recombine.
Rule 1: Linkage — Morgan crossed yellow-bodied white-eyed with brown-bodied red-eyed flies. The two genes were close together on the X chromosome, so the F₂ ratio deviated from 9 : 3 : 3 : 1 and parental combinations were much more common than recombinants.
- Easy : Linked genes are inherited together.
- Medium : Linkage is shown by too few recombinants.
- Exam-type : Q: Why did the F₂ of Morgan's cross not show 9 : 3 : 3 : 1? A: The two genes were linked on the same chromosome.
Rule 2: Recombination and gene mapping — Crossing over between homologous chromosomes in meiosis separates linked genes and produces recombinants. Genes far apart show a higher recombination frequency. Sturtevant used these frequencies as the distance between genes to make linkage maps.
- Easy : Recombination frequency measures the distance between genes.
- Medium : Higher recombination = genes farther apart.
- Exam-type : Q: Who used the recombination frequency to map genes? A: Alfred Sturtevant.
Tightly linked genes show very few recombinants. Morgan's work gave the chromosome theory its proof; Drosophila is useful because of a short life cycle and many hereditary variations that can be seen with a low-power microscope.
Saying linked genes never separate. Crossing over can separate them, though rarely if they are close.
Memory clue: Close → Linked — The closer two genes are, the more linked they are, the fewer the recombinants.
Spot the difference
| Point | Linkage | Recombination |
|---|---|---|
| Meaning | Genes inherited together | Formation of new gene combinations |
| Cause | Genes on the same chromosome | Crossing over |
| Effect on ratio | Parental types are more | Recombinants appear |
Practice questions
- 1Who proposed the chromosomal theory of inheritance?
- 2Which organism did Morgan use?
- 3What is linkage?
- 4Name the process that creates new gene combinations on homologous chromosomes.
- 5Who prepared the first gene map?
- 6Give one reason for choosing Drosophila for genetic studies.
- 7Two genes show 2 per cent recombination and two others show 20 per cent. Which pair is closer?
- 8How did Morgan's results support Sutton and Boveri?
Show answers (try them yourself first)
- 1Sutton and Boveri.
- 2The fruit fly, Drosophila melanogaster.
- 3Inheritance of genes together because they lie on the same chromosome.
- 4Crossing over (recombination).
- 5Alfred Sturtevant.
- 6Short life cycle, many progeny and clear hereditary variations.
- 7The pair with 2 per cent recombination.
- 8They showed that genes are carried on chromosomes and behave as one unit when linked.
Polygenic inheritance and pleiotropy
Skin colour and height vary smoothly, with many shades and heights in between, not just two types. A single gene can also affect many body features at once.
In polygenic inheritance a trait is controlled by three or more genes; each dominant allele adds to the effect (human skin colour, height). In pleiotropy a single gene affects more than one trait (phenylketonuria affects mental ability, hair and skin colour).
In the figure : One kind has many genes for one trait; the other has one gene for many traits.
Rule 1: Polygenic traits — Each dominant allele contributes to the trait, so with more dominant alleles the trait is stronger. A cross of two heterozygotes for three genes gives a range of shades with the most common in the middle (bell-shaped curve).
- Easy : More dominant alleles = darker skin.
- Medium : Phenotype depends on the number of alleles, not on which gene.
- Exam-type : Q: Give an example of a polygenic trait. A: Human skin colour (or height).
Rule 2: Pleiotropy — The gene for phenylketonuria is on an autosome; a mutation in the gene makes an enzyme defective and causes mental retardation, reduced hair and skin pigmentation together. Starch synthesis gene in pea seeds also influences seed shape.
- Easy : PKU is a pleiotropic disorder.
- Medium : One defective gene can cause several symptoms.
- Exam-type : Q: Define pleiotropy. A: A single gene controlling or affecting multiple phenotypic traits.
Polygenic = many genes → one trait (continuous variation). Pleiotropy = one gene → many traits.
Writing that pleiotropy means many genes act on one trait. That is polygenic inheritance.
Memory clue: Poly → many genes, Pleio → many effects — Polygenic = many genes; Pleiotropy = many effects.
Spot the difference
| Point | Polygenic | Pleiotropy |
|---|---|---|
| Genes involved | Many | One |
| Traits affected | One | Many |
| Example | Skin colour | Phenylketonuria |
Practice questions
- 1What is polygenic inheritance?
- 2Give one example of pleiotropy.
- 3Name one polygenic human trait.
- 4What is the nature of variation in polygenic traits?
- 5How many genes are involved in pleiotropy?
- 6What kind of curve does polygenic inheritance usually give?
- 7Why do children of tall parents sometimes have different heights?
- 8Why is phenylketonuria an example of pleiotropy?
Show answers (try them yourself first)
- 1Control of one trait by three or more genes.
- 2Phenylketonuria (or the starch synthesis gene in peas).
- 3Skin colour (or height).
- 4Continuous.
- 5One gene.
- 6A bell-shaped curve.
- 7Height is polygenic; different combinations of alleles from the parents give different effects.
- 8The mutation of one gene has several effects: mental retardation and pigment changes.
Sex determination
The sex of a human baby is decided at the moment of fertilisation, by the sperm.
In humans, 22 pairs of chromosomes are autosomes and one pair is the sex chromosomes: XX in females and XY in males. This is the XY type (also in Drosophila); grasshoppers show XO type (male has one X); birds show ZW type (female ZW, male ZZ). In honey bees it is haplodiploid.
In the figure : Half the sperm carry X, half carry Y, so the chance of a boy or a girl is 50 per cent each.
In the figure : In bees the sex depends on the number of chromosome sets.
Rule 1: Humans — A woman produces only one type of egg (22 + X), a man two kinds of sperm (22 + X and 22 + Y). The sex of the child depends on whether the egg is fertilised by an X-bearing or Y-bearing sperm.
- Easy : X sperm + egg gives a girl.
- Medium : The sperm decides the sex of the baby.
- Exam-type : Q: Which parent decides the sex of the child? A: The father.
Rule 2: Other animals — In birds the female is ZW and decides the sex. In honey bees an unfertilised egg becomes a haploid male drone; a fertilised egg becomes a diploid female (queen or worker). Drones make sperms by mitosis and have no father.
- Easy : Birds: male ZZ, female ZW.
- Medium : A drone has 16 chromosomes.
- Exam-type : Q: How do drones develop? A: From unfertilised eggs by parthenogenesis.
Humans XX/XY; birds ZZ/ZW (female is heterogametic); grasshopper XO. Bees: queen and worker diploid (32), drone haploid (16).
Saying the mother decides the baby's sex in humans. In humans the egg always carries X; the sperm decides.
Memory clue: W for Woman bird — In birds the W is in the female: ZW female, ZZ male.
Spot the difference
| Point | Humans | Birds |
|---|---|---|
| Female | XX | ZW |
| Male | XY | ZZ |
| Heterogametic sex | Male | Female |
Practice questions
- 1What are autosomes?
- 2What are the sex chromosomes of a human female?
- 3Which sex is ZW in birds?
- 4What is the chromosome number of a honey bee drone?
- 5Name the type of sex determination in grasshoppers.
- 6What fraction of sperm carry the Y chromosome?
- 7Why is the chance of a boy or a girl equal in humans?
- 8Why does a drone have no father?
Show answers (try them yourself first)
- 1Chromosomes other than the sex chromosomes.
- 2XX.
- 3Female.
- 416 (haploid).
- 5XO type.
- 6Half (50 per cent).
- 7Equal numbers of X-bearing and Y-bearing sperm are produced, and either is equally likely to fertilise the egg.
- 8It arises from an unfertilised egg, so only the queen contributes genes.
Mutation and pedigree analysis
A single changed letter in the instructions for haemoglobin is enough to change the shape of a red blood cell. Families can trace such a change through a family tree.
A mutation is a sudden, inheritable change in the DNA. A point mutation changes a single base pair (sickle-cell anaemia). Loss or gain of a whole set of chromosomes is polyploidy; of one or a few chromosomes is aneuploidy. A pedigree is a family tree used to trace a trait through generations.
In the figure : Mutations may be small (one base) or large (whole chromosomes).
In the figure : Common pedigree symbols: circle female, square male, shaded affected.
Rule 1: Mutation — Mutations arise from chemical changes in DNA or from mutagens such as UV rays, X-rays and certain chemicals. Sickle-cell anaemia is a point mutation in the gene for the β-globin chain of haemoglobin.
- Easy : Mutation = change in DNA.
- Medium : UV radiation is a mutagen.
- Exam-type : Q: What is a point mutation? A: A change of a single base pair in DNA.
Rule 2: Pedigree analysis — Studying the family tree shows whether a trait is dominant or recessive, and whether it is sex-linked. It is useful because human beings cannot be crossed at will in experiments.
- Easy : A pedigree is a family tree.
- Medium : Pedigree helps in genetic counselling.
- Exam-type : Q: Why is pedigree analysis needed in humans? A: Controlled crosses are not possible, so family records are used to study inheritance.
Mutation = inheritable change in DNA. Aneuploidy: 2n ± 1 (e.g., trisomy 21); polyploidy: 3n, 4n.
Thinking all mutations are harmful. Some are neutral or even useful for the organism.
Memory clue: A-N-eu vs P-oly — Aneuploidy = a Number off by one; Polyploidy = Plenty of sets.
Spot the difference
| Point | Aneuploidy | Polyploidy |
|---|---|---|
| Change | One or a few chromosomes | Complete sets |
| Example | Down syndrome | Triploid or tetraploid plants |
| Number | 47 or 45 in humans | 3n, 4n |
Practice questions
- 1Define mutation.
- 2Give one mutagen.
- 3Which disease is caused by a point mutation in the β-globin gene?
- 4What is a pedigree?
- 5What is aneuploidy?
- 6What is polyploidy?
- 7Why do geneticists use pedigrees for human traits?
- 8Is every mutation passed to the next generation? Explain.
Show answers (try them yourself first)
- 1A sudden, inheritable change in the genetic material.
- 2UV rays (or X-rays, chemicals).
- 3Sickle-cell anaemia.
- 4A family tree used to trace the inheritance of a trait.
- 5Gain or loss of one or a few chromosomes.
- 6Gain of complete sets of chromosomes.
- 7Experimental crosses are not possible in humans.
- 8No; only mutations in gametes (or germ-line cells) can be inherited.
Genetic disorders
Colour blindness, haemophilia, thalassemia and Down syndrome are all caused by changes in genes or chromosomes, and some run in families.
Genetic disorders are of two kinds. Mendelian disorders are caused by a mutation in a single gene: haemophilia and colour blindness (X-linked recessive), sickle-cell anaemia, phenylketonuria and thalassemia (autosomal recessive). Chromosomal disorders are caused by absence or excess of chromosomes: Down syndrome, Klinefelter's syndrome and Turner's syndrome.
In the figure : Mendelian disorders: gene level; chromosomal disorders: whole chromosomes.
Rule 1: Mendelian disorders — Haemophilia: blood does not clot properly; carrier women pass it on to sons; a female is haemophilic only if both X chromosomes carry the gene. Colour blindness: red-green colour blindness in about 8 per cent of males and 0.4 per cent of females. Sickle-cell anaemia: a change from glutamic acid to valine in the β-globin chain makes red cells sickle-shaped. Phenylketonuria: the enzyme that converts phenylalanine to tyrosine is missing. Thalassemia: reduced rate of synthesis of α or β globin chains.
- Easy : Haemophilia is X-linked recessive.
- Medium : A carrier mother and a normal father: about half of the sons may be affected.
- Exam-type : Q: What is the difference between sickle-cell anaemia and thalassemia? A: Sickle-cell: structural defect in the globin molecule; thalassemia: reduced quantity of a globin chain.
Rule 2: Chromosomal disorders — Down syndrome: trisomy 21 gives 47 chromosomes. Klinefelter's: 47 chromosomes with XXY, male with underdeveloped testes and breast development. Turner's: 45 chromosomes with XO, a short female with rudimentary ovaries. They arise from non-disjunction of chromosomes at meiosis.
- Easy : Down syndrome = 47 chromosomes.
- Medium : Turner: XO female, 45 chromosomes.
- Exam-type : Q: What is non-disjunction? A: Failure of chromosomes to separate during meiosis, giving gametes with extra or missing chromosomes.
Down 21 (47), Klinefelter XXY (47), Turner XO (45). Haemophilia and colour blindness are X-linked recessive; sickle-cell, PKU and thalassemia are autosomal recessive.
Thinking Klinefelter's syndrome occurs in females. It occurs in males (XXY); Turner's occurs in females (XO).
Memory clue: K → Kings (males), T → Tiny (females) — Klinefelter = XXY male; Turner = XO female.
Spot the difference
| Point | Klinefelter's | Turner's |
|---|---|---|
| Sex chromosomes | XXY | XO |
| Total chromosomes | 47 | 45 |
| Individual | Male | Female |
Practice questions
- 1What is a Mendelian disorder?
- 2Name one X-linked recessive disorder.
- 3What is the chromosome number in Down syndrome?
- 4What is the sex chromosome pattern of Turner's syndrome?
- 5Name the pigment-related defect in phenylketonuria.
- 6What causes sickle-cell anaemia?
- 7Why do haemophilia and colour blindness appear more often in males?
- 8Why is a haemophilic daughter very rare?
Show answers (try them yourself first)
- 1A disorder caused by an alteration in a single gene.
- 2Haemophilia (or colour blindness).
- 347 (trisomy of chromosome 21).
- 4XO.
- 5Lack of the enzyme that changes phenylalanine to tyrosine, which leads to reduced pigmentation.
- 6Substitution of glutamic acid by valine at the sixth position of the β-globin chain.
- 7The recessive gene is on the X chromosome and males have only one X, so one allele is enough to show the disease.
- 8She must receive the gene on both X chromosomes, from a haemophilic father and a carrier or affected mother.