Exercises
Mention the advantages of selecting pea plant for experiment by Mendel.
Answer
(i) Garden pea has many clearly contrasting forms of easily observed traits (e.g. tall/dwarf stem, round/wrinkled seed, yellow/green seed, violet/white flower). (ii) Its flowers are bisexual and normally self-pollinate, so true-breeding lines could be built by continuous self-pollination over generations. (iii) Cross-pollination can be done artificially by emasculation and bagging. (iv) It is an annual with a short life cycle, so many generations can be studied quickly. (v) Each cross yields many seeds, giving large samples for statistical analysis. (vi) It is easy to grow.
Differentiate between the following – (a) Dominance and Recessive (b) Homozygous and Heterozygous (c) Monohybrid and Dihybrid.
Answer
(a) A dominant allele (factor) expresses itself in the phenotype even in the heterozygous condition (e.g. T for tall in Tt). A recessive allele expresses itself only in the homozygous condition (tt); in a heterozygote its effect is masked.
(b) A homozygous individual has two identical alleles of a gene (TT or tt) and is true-breeding, producing one type of gamete. A heterozygous individual has two different alleles (Tt) and produces two types of gametes (T and t).
(c) A monohybrid cross studies the inheritance of one pair of contrasting characters (e.g. tall × dwarf); F₂ phenotypic ratio 3:1, genotypic 1:2:1. A dihybrid cross studies two pairs of contrasting characters together (e.g. seed colour and shape); F₂ phenotypic ratio 9:3:3:1.
A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
Answer
Number of types of gametes = 2ⁿ, where n = number of heterozygous loci (assuming independent assortment)
⇒ 2⁴ = 16 types of gametes.
Explain the Law of Dominance using a monohybrid cross.
Answer
Law of Dominance: (i) characters are controlled by discrete units called factors; (ii) factors occur in pairs; (iii) in a dissimilar pair of factors, one member (dominant) expresses itself and the other (recessive) does not.
Cross: true-breeding tall (TT) × true-breeding dwarf (tt)
⇒ gametes T and t
⇒ F₁ all Tt, all tall. Though F₁ carries t, only tallness appears, so T is dominant over t. On selfing F₁ (Tt × Tt), F₂ gives TT : Tt : tt = 1 : 2 : 1, i.e. tall : dwarf = 3 : 1. The reappearance of dwarf plants in F₂ shows the recessive factor was present but masked in F₁.
Define and design a test-cross.
Answer
A test cross is a cross between an organism showing the dominant phenotype (whose genotype is unknown) and the recessive parent (homozygous recessive), used to find out the genotype of the dominant individual.
Design: a tall pea plant (TT or Tt?) × dwarf plant (tt).
If the tall plant is TT: TT × tt
⇒ all Tt
⇒ all progeny tall.
If the tall plant is Tt: Tt × tt
⇒ Tt : tt = 1 : 1
⇒ half tall, half dwarf.
(For a dihybrid test cross, TtYy × ttyy gives 1:1:1:1.)
Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
Answer
Take height in pea, T (tall) dominant over t (dwarf). Heterozygous male = Tt, gametes T and t.
Case 1: homozygous dominant female TT (gametes T). Punnett square: T × T
⇒ TT; T × t
⇒ Tt. F₁ = 1 TT : 1 Tt
⇒ all tall (100% dominant phenotype).
Case 2: homozygous recessive female tt (gametes t). Punnett square: t × T
⇒ Tt; t × t
⇒ tt. F₁ = 1 Tt : 1 tt
⇒ 50% tall : 50% dwarf (phenotypic ratio 1:1).
When a cross in made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be (a) tall and green. (b) dwarf and green.
Answer
Treat each gene separately. Tt × Tt
⇒ 3/4 tall, 1/4 dwarf. Yy × yy
⇒ 1/2 yellow (Yy), 1/2 green (yy).
(a) Tall and green = 3/4 × 1/2 = 3/8.
(b) Dwarf and green = 1/4 × 1/2 = 1/8.
(Gametes: TtYy
⇒ TY, Ty, tY, ty; Ttyy
⇒ Ty, ty. A 4 × 2 Punnett square of 8 boxes gives 3 tall yellow : 3 tall green : 1 dwarf yellow : 1 dwarf green.)
Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dibybrid cross?
Answer
If the two genes are linked on the same chromosome, they do not assort independently, so the 9:3:3:1 ratio is not obtained. Suppose both parents are AB/ab (dominant alleles together on one chromosome). With complete linkage, each parent forms only two types of gametes, AB and ab, in equal numbers. Offspring: 1 AB/AB : 2 AB/ab : 1 ab/ab
⇒ phenotypic ratio 3 (both dominant traits) : 1 (both recessive traits), i.e. only parental combinations appear. If linkage is incomplete, crossing over produces a small number of recombinant gametes (Ab, aB), so a few non-parental phenotypes also appear, but parental types remain far more frequent than the 9:3:3:1 expectation; the proportion of recombinants depends on how close the genes are.
Briefly mention the contribution of T.H. Morgan in genetics.
Answer
(i) Morgan experimentally verified the chromosomal theory of inheritance using the fruit fly Drosophila melanogaster. (ii) Through dihybrid crosses with sex-linked genes (e.g. yellow body/white eye), he found that genes on the same chromosome do not segregate independently; he coined the terms linkage (physical association of genes on a chromosome) and recombination (generation of non-parental combinations). (iii) He showed that some genes are tightly linked (white and yellow, 1.3% recombination) while others are loosely linked (white and miniature wing, 37.2%). (iv) His work led his student Sturtevant to use recombination frequency to construct genetic (chromosome) maps.
What is pedigree analysis? Suggest how such an analysis, can be useful.
Answer
Pedigree analysis is the study of the inheritance of a particular trait or disorder over several generations of a family, using a family tree drawn with standard symbols. Since controlled crosses are not possible in humans, it is the main way to study human inheritance. Uses: (i) it shows whether a trait is dominant or recessive and autosomal or sex-linked; (ii) it identifies carriers; (iii) it helps predict the chance of a disorder (e.g. haemophilia, sickle-cell anaemia, colour blindness) in future children, which is valuable in genetic counselling and in avoiding marriages between carriers or close relatives.
How is sex determined in human beings?
Answer
Humans have the XY type of sex determination. Of 23 pairs of chromosomes, 22 pairs are autosomes and one pair is sex chromosomes. Females are XX (homogametic) and produce only X-carrying eggs. Males are XY (heterogametic) and produce two kinds of sperms in equal numbers: 50% carry X and 50% carry Y (besides 22 autosomes). If an X-bearing sperm fertilises the egg, the zygote is XX and develops into a female; if a Y-bearing sperm does, the zygote is XY and develops into a male. So the sperm decides the sex of the child, and each pregnancy has a 50% chance of a boy or a girl.
A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
Answer
Blood group O has genotype ii, so the child got one i from each parent. Hence father (group A) = Iᴬi and mother (group B) = Iᴮi.
Cross: Iᴬi × Iᴮi
⇒ gametes Iᴬ, i and Iᴮ, i
⇒
IᴬIᴮ (group AB) : Iᴬi (group A) : Iᴮi (group B) : ii (group O) = 1 : 1 : 1 : 1.
So other children may have any of these four genotypes, i.e. blood groups AB, A, B or O.
Explain the following terms with example (a) Co-dominance (b) Incomplete dominance
Answer
(a) Co-dominance: both alleles of a gene express themselves fully in the heterozygote, so F₁ shows both parental characters. Example: ABO blood groups – alleles Iᴬ and Iᴮ are co-dominant; a person with IᴬIᴮ makes both A and B sugars on RBCs and has blood group AB.
(b) Incomplete dominance: neither allele is completely dominant, so the heterozygote shows an intermediate phenotype. Example: snapdragon (Antirrhinum) flower colour – red (RR) × white (rr)
⇒ F₁ all pink (Rr); F₂ gives red : pink : white = 1 : 2 : 1, so phenotypic and genotypic ratios are the same.
What is point mutation? Give one example.
Answer
A point mutation is a change (substitution) of a single base pair in DNA. Example: sickle-cell anaemia, where a single base change in the sixth codon of the β-globin gene (GAG
⇒ GUG) replaces glutamic acid by valine at the sixth position of the β-globin chain.
Who had proposed the chromosomal theory of the inheritance?
Answer
Walter Sutton and Theodore Boveri (1902). Sutton united chromosomal segregation with Mendel's principles and called it the chromosomal theory of inheritance; T.H. Morgan later verified it experimentally.
Mention any two autosomal genetic disorders with their symptoms.
Answer
(i) Sickle-cell anaemia (autosomal recessive; HbˢHbˢ): mutant haemoglobin polymerises under low oxygen tension, changing RBCs from biconcave discs to elongated sickle shapes, causing anaemia.
(ii) Down's syndrome (trisomy of chromosome 21, 47 chromosomes): short stature, small round head, furrowed tongue and partially open mouth, broad palm with a characteristic palm crease, and retarded physical, psychomotor and mental development.
(Other examples: phenylketonuria – lack of the enzyme converting phenylalanine to tyrosine, causing mental retardation; thalassemia – reduced synthesis of α or β globin chains, causing anaemia.)
Important Extra Questions
State Mendel's Law of Segregation.
Answer
The two alleles of a gene do not blend; they separate during gamete formation so that each gamete receives only one of the two factors.
Why is haemophilia more common in males than in females?
Answer
It is an X-linked recessive disorder. Males have one X, so a single recessive allele causes the disease; females need two copies (homozygous), which is rare.
Give the karyotype of Klinefelter's and Turner's syndromes.
Answer
Klinefelter's: 47, XXY (sterile male with gynaecomastia). Turner's: 45, X0 (sterile female with rudimentary ovaries).
How is sex determined in birds?
Answer
By ZW type: females are heterogametic (ZW) and males homogametic (ZZ), so the egg decides the sex.
What is polygenic inheritance? Give an example.
Answer
A trait controlled by three or more genes, each adding a small effect, giving continuous variation; e.g. human skin colour, height.