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.
Discuss in detail the contributions of Morgan and Sturvant in the area of genetics.
T H Morgan (1866-1945) was given the Nobel Prize in 1933.
His contributions are
(i) Morgan worked on fruit fly Drosophila melanogaster and proposed the chromosomal theory of linkage.
(ii) He stated and established that genes are located on the chromosome.
(iii) He established the principle of linkage, crossing over, sex-linked inheritance and discovered the relation between gene and chromosome.
(iv) He established the technique of chromosome mapping.
(v) He observed and worked on mutation.
Alfred Henry Sturtevant (1891-1970) student of morgan was given the National Medal of Science in 1967. His contributions are
(i) He constructed the first genetic map of a chromosome while working on the Drosophila genome.
(ii) His main contributions to science include his analysis of genetic 'linkage groups,' which became classical method of chromosome mapping that is still used today. In 1913, he determined that genes were arranged on chromosomes in a linear fashion, like beads on a necklace. He also showed that the gene for any specific trait was in a fixed location (locus).
(iii) His work on Drosophila proved that two closely related species showed newly recurring mutations that were allelic and thus probably identical. His work also helped to determine genetic role in sexual selection and development and displayed the importance of chromosomal crossing over in mutations.
(iv) One of Sturtevant's principal contributions was his introduction to the concept that the frequency of crossing over between two genes could help to determine their proximity on a linear genetic map. His experiments determined that the frequency of double crossing over can be used to deduce gene order.