Principles of Inheritance and Variation: common doubts

Question 1

Why did Mendel choose the garden pea for his experiments?

Accepted Answer

The pea was convenient: it has many easily seen contrasting characters, a short life cycle, and produces many seeds. It is normally self-pollinating, so true-breeding lines are easy to obtain, yet it can be cross-pollinated by hand after removing the anthers. These qualities let him follow clear ratios across generations.

Question 2

What is the difference between genotype and phenotype?

Accepted Answer

Genotype is the genetic make-up of an individual, written with allele symbols such as Tt. Phenotype is the visible or measurable expression of that make-up, such as tall. Different genotypes can give the same phenotype, as TT and Tt are both tall, so a phenotype alone does not reveal the genotype.

Question 3

What is the difference between the phenotypic and genotypic ratio in a monohybrid cross?

Accepted Answer

In the F₂ of a monohybrid cross with complete dominance, the phenotypic ratio is 3:1 and the genotypic ratio is 1:2:1. Of every four plants, three show the dominant trait, but only one of those three is homozygous dominant; the other two are heterozygous. The two ratios become identical only when the heterozygote looks different from both homozygotes, as in incomplete dominance or co-dominance.

Question 4

What is a test cross and what ratio does it give?

Accepted Answer

A test cross is crossing an individual of dominant phenotype with a homozygous recessive parent to find out its genotype. If the individual is heterozygous, the offspring appear in a 1:1 ratio of dominant to recessive. If it is homozygous dominant, all offspring show the dominant trait. A dihybrid test cross gives 1:1:1:1.

Question 5

What does the law of segregation say?

Accepted Answer

The law of segregation says the two alleles of a gene separate from each other during gamete formation, so each gamete carries only one allele. The alleles come together again at fertilisation. It explains why the recessive trait reappears in the F₂, and it rests on the alleles being pure and not blended in the heterozygote.

Question 6

What is the difference between incomplete dominance and co-dominance?

Accepted Answer

In incomplete dominance the heterozygote shows an intermediate phenotype, as pink snapdragon flowers from red and white parents. In co-dominance both alleles are fully expressed in the heterozygote, as with the IA and IB alleles in blood group AB. One blends the effect and the other shows both effects side by side.

Question 7

How many genotypes and phenotypes does the ABO blood group system have?

Accepted Answer

It has six genotypes and four phenotypes. The genotypes are IAIA, IAi, IBIB, IBi, IAIB and ii. The phenotypes are the blood groups A, B, AB and O. Three alleles control it, but any person carries only two, and IA and IB are dominant over i while being co-dominant to each other.

Question 8

Why is the ABO system an example of multiple alleles?

Accepted Answer

Because the gene I has more than two allelic forms in the population: IA, IB and i. A single individual still has only two of them, one on each homologous chromosome. Multiple alleles can be seen only when many individuals of the population are studied, not in one person.

Question 9

What happens in the F₂ of a snapdragon cross of red and white flowers?

Accepted Answer

The F₂ has red, pink and white flowers in a 1:2:1 ratio. Here the phenotypic and genotypic ratios are identical, because the heterozygote (pink) is distinguishable from both homozygotes. The F₁ is all pink, yet pure red and white plants reappear in the F₂, which shows that the factors themselves never blended; only their combined expression in the heterozygote was intermediate.

Question 10

What does the law of independent assortment say?

Accepted Answer

It says that when two pairs of traits are considered in a cross, the alleles of each gene separate independently of the other pair during gamete formation. A dihybrid F₂ therefore shows 9:3:3:1, which is the product of two separate 3:1 ratios. It holds for genes on different chromosomes and not for tightly linked ones.

Question 11

Which Mendelian dihybrid ratio belongs to which phenotype?

Accepted Answer

In a cross of round yellow with wrinkled green seeds, the F₂ is 9 round yellow, 3 round green, 3 wrinkled yellow and 1 wrinkled green. The 9 and the 1 are the parental combinations. The 3s are the new combinations of traits, which shows that the two traits are inherited independently.

Question 12

What is the chromosomal theory of inheritance?

Accepted Answer

It states that genes are located on chromosomes and that the behaviour of chromosomes in meiosis explains Mendel's laws. Sutton and Boveri noticed the parallel, and Morgan later verified it experimentally in the fruit fly, Drosophila. Chromosomes pair and then separate, just like Mendelian factors, which explains segregation and independent assortment.

Question 13

What is linkage and how does it affect recombination?

Accepted Answer

Linkage is the tendency of genes on the same chromosome to be inherited together. The closer two genes lie, the less often they are separated by crossing over, so recombination frequency is low. Morgan found 1.3 per cent between yellow body and white eye but 37.2 per cent between white eye and miniature wing.

Question 14

How is recombination frequency used to map genes?

Accepted Answer

Alfred Sturtevant, Morgan's student, treated the percentage of recombinants between two genes as their distance apart on the chromosome: the larger the percentage, the farther apart. Putting many such pairwise distances together let him arrange genes in order along the chromosome, producing the first genetic maps.

Question 15

What is the difference between polygenic inheritance and pleiotropy?

Accepted Answer

In polygenic inheritance several genes together control one trait, as in human skin colour, with each dominant allele adding to the effect. In pleiotropy a single gene affects several traits, as the phenylketonuria gene does, causing mental retardation along with reduced hair and skin pigmentation. One is many genes for one trait and the other is one gene for many traits.

Question 16

How is sex determined in birds, and how is it different from humans?

Accepted Answer

In birds the female is heterogametic (ZW) and the male is homogametic (ZZ), so the egg decides the sex. In humans the male is XY and the female XX, so the sperm decides the sex. The roles of the two sexes are simply reversed between the systems.

Question 17

How is sex determined in honey bees?

Accepted Answer

Honey bees show haplodiploidy. The queen and workers (females) are diploid with 32 chromosomes and develop from fertilised eggs. Drones (males) are haploid with 16 chromosomes and develop from unfertilised eggs by parthenogenesis. So the sex depends on the number of chromosome sets rather than on sex chromosomes.

Question 18

Why are X-linked recessive disorders like haemophilia more common in males?

Accepted Answer

Males have only one X chromosome, so a single recessive allele on it is expressed. A female needs two copies, and she is usually a carrier instead. A carrier mother passes the allele to half of her sons, while sons never inherit it from the father since they receive his Y. Colour blindness works the same way.

Question 19

What is the molecular cause of sickle-cell anaemia?

Accepted Answer

A single base substitution in the beta-globin gene changes GAG to GUG in the mRNA, replacing glutamic acid by valine at the sixth position of the beta chain. This makes haemoglobin polymerise under low oxygen and the red cells take a sickle shape. It is an autosomal recessive disorder, with homozygous individuals showing the disease.

Question 20

What is the difference between sickle-cell anaemia and thalassemia?

Accepted Answer

Sickle-cell anaemia is a qualitative defect, because the globin made is structurally abnormal. Thalassemia is a quantitative defect, because too little normal globin is made. Both are autosomal recessive. Alpha thalassemia involves genes on chromosome 16 and beta thalassemia involves a gene on chromosome 11.

Question 21

What chromosomal changes cause Down's, Klinefelter's and Turner's syndromes?

Accepted Answer

Down's syndrome is trisomy of chromosome 21, giving 47 chromosomes. Klinefelter's syndrome is 47 with XXY, an otherwise male individual with an extra X. Turner's syndrome is 45 with a single X (X0), a female lacking one X. The Klinefelter's and Turner's individuals are both sterile.

Question 22

What is the difference between aneuploidy and polyploidy?

Accepted Answer

Aneuploidy is the gain or loss of one or a few chromosomes, caused by failure of chromatids to separate during anaphase, as in Down's syndrome. Polyploidy is an increase in whole sets of chromosomes, caused by failure of cytokinesis after telophase. So one changes the count by a few chromosomes and the other by entire sets.

Principles of Inheritance and Variation: common doubts, answered

Mendel's laws of inheritance