With the help of an example differntiate between incomplete dominance and co-dominance.
Incomplete dominance is a phenomenon where two contrasting alleles are present together but neither of the alleles is dominant over other and the phenotype formed is intermediate of the two alleles.
e.g., the kind of inheritance in the dog flower (Snapdragon or Antirrhinum species) in which the intermediate trait is expressed in $\mathrm{F}_1$-generation.
Codominance is a phenomenon in which when two contrasting alleles are present together and both of the alleles express themselves.
e.g., AB blood group in humans where both the alleles are expressed to produce RBC surface antigens $A$ and $B$.$$ \text { (i) Coss showing incomplete dominance } $$
$$ \text { (ii) Blood group showing co-dominance } $$
| Genotype | Surface Antigen | $$ \text { Blood Group } $$ |
|---|---|---|
| $\mathrm{I}^{\mathrm{A} i}$ (dominance) | A | A |
| $\left.\mathrm{I}^{\mathrm{A}}\right|^{\mathrm{A}}$ | A | A |
| $\mathrm{I}^{\mathrm{B}} \mathrm{i}$ (dominance) | B | B |
| $\mathrm{I}^{\mathrm{B}} \mathrm{I}^{\mathrm{B}}$ | B | B |
| $\mathrm{I}^{\mathrm{A}} \mathrm{I}^{\mathrm{B}}$ (co-dominance) | AB | AB |
| ii | - | O |
It is said, that the harmful alleles get eliminated from population over a period of time, yet sickle-cell anaemia is persisting in human population. Why?
Sickle-cell anaemia is an autosomal recessive disease caused by haemoglobins an oxygen carrying protein in blood cells.
Despite the disease's lethal symptoms, it protects the carrier from malaria. Its allele are most common in the people of African descent (about 7\% people of African descent carry an allele) and some other are as where malaria in prevalent.
It provides the vital protection from malaria. Individuals with HbAS heterozygotes tend to survive better than individuals with HbSS (homozygotes) as they are not exposed to the same severity of risk.
In a plant tallness is dominant over dwarfness and red flower is dominant over white. Starting with the parents work out a dihybrid cross. What is standard dihybrid ratio? Do you think the values would deviate if the two genes in question are interacting with each other?
The standard dihybrid ratio is $9: 3: 3: 1$. Yes, the values will show deviation if the two genes in the above case are interacting with each other. When the genes are linked, they do not assort independently but remain together in the gametes and the offsprings, give a dihybrid ratio of $3: 1$ and show a test cross ratio of $1: 1$ instead of $1: 1: 1: 1$.
(a) In humans, males are heterogametic and females are homogametic, Explain. Are there any examples where males are homogametic and females heterogametic?
(b) Also describe as to, who deterrmines the sex of an unborn child? Mention whether temperature has a role in sex determination.
(a) The term homogametic and heterogametic refers to the organism depending upon whether all the gametes contain one type of sex chromosome (homo same) or two different types of sex chromosomes (hetero different).
Humans show XX/XY type of sex determination, i.e., females contain 2 copies of X -chromosome and males contain 1 X and 1 Y -chromosome. Therefore, ova produced by females contain the same sex chromosome, i.e., X .
On the other hand the sperms contain 2 different types of chromosomes, i.e., $50 \%$ sperms have $X$ and $50 \%$ have $Y$-chromosomes (meiosis). Therefore, the sperms are different with respect to the composition of sex chromosome.
In case of humans, females are considered to be homogametic while males are heterogametic. Yes, there are examples where males are homogametic and females are heterogametic. In some birds the mode of sex determination is denoted by ZZ (males) and ZW (females). Certain moths and butterflies also show homogametic males and heterogametic females.
(b) As a rule the heterogametic organism determines the sex of the unborn child. In case of humans, since males are heterogametic it is the father and not the mother who decides the sex of the child. In some animals like crocodiles, lower temperature favour hatching of female offsprings and higher temperatures lead to hatching of male offsprings.
A normal visioned woman, whose father is colour blind, marries a normal visioned man. What would be probability of her sons and daughters to be colour blind? Explain with the help of a pedigree chart.
The genotype of parents are
$50 \%$ daughters are normal visioned but $50 \%$ will be carries and $50 \%$ of sons are likely to be colour blind and 50\% are normal visioned.