Principles of Inheritance & Variation Test 9

Down’s syndrome increases if the age of the mother exceeds forty years because age adversely affects meiotic chromosome behaviour. Meiosis in the egg cells is not completed, until after fertilization. During this long gap (till meiosis is not completed) egg cells are arrested in prophase I and chromosomes are unpaired. The greater the time they remain unpaired greater the chance for unpairing and chromosome non-disjunction.

Linkage group is linearly arranged groups of genes which fail to show independent assortment as these are present on the same chromosomes.

Haemophilia is a sex-linked recessive disorder in which X-chromosome has the haemophilic gene. A haemophilic father can never pass the gene for haemophilia to his son.

Abnormal chromosomal behaviour in insects can result in the formation of gynandromorphs or sexual mosaics in which some parts of the animal are female and other parts are male. Some gynandromorphs in Drosophila are bilateral intersex. Bilateral gynandromorphs have been explained on the basis of an irregularity in mitosis at the first cleavage of the zygote. Infrequently a chromosome lags in division and does not arrive at the pole in time to be included in the reconstructed nucleus. When one of the X chromosomes of an XX (female) zygote lags in the spindle, one daughter nucleus receives two X chromosomes. A mosaic body pattern is thus established. One nucleus in. the nuclei stage would be XX (female) and one would be XO (male).

on the same chromosome while unlinked genes are the ones found on different chromosomes. Linked gene show dihybrid ratio of 3 :1. Linked genes do not show independent assortment.

Colour blindness is X-linked sex inheritance. The frequency of red-green colour blindness is many times higher in males than that in the females because males have only one X chromosomes therefore they develop colour blindness when their sole X- chromosome has the allele for it. Thus, males are more prone to colour blindness while females are carriers. For becoming colour blind, the female must have the allele for it in both her X-chromosomes and if only one X-chromosome of female possess allele for colour blind character she becomes the carrier for this characteristic.

In mammals, the presence of a Y chromosome is required for the development of a male sex phenotype. In contrast, the Y chromosome plays no significant role in sex determination in Drosophila. A Y chromosome is required for maleness, moreover, the presence of a single Y chromosome is sufficient even in the presence of several X chromosomes (e.g XXXXY).

Sickle-cell anaemia is caused by a single point mutation in the beta haemoglobin gene which converts a GAG, codon into GUG, which code for valine amino acid rather than glutamic acid.

Karyotype is the chromosome complement of a cell, individual or group of similar individuals that provides description of number, types and other characteristics of chromosomes. Any change in the chromosome number is detected immediately. Chromosomal aberrations or abnormalities are found about. This chromosome technique has made it possible to know the sex of developing fetus by drawing amniotic fluid and preparing karyotypes from cells derived from fetus floating in this fluid.

Crossing F₁ with recessive parent will. may inherit recessive genotypic characters as dominant characters in their offspring.

Haplodiploidy is a type of sex determination in which the male is haploid while the female is diploid. It occur in some insects like bees, ants and wasps. Male insects are haploid because they develop parthenogenetically from unfertilised eggs. Meiosis

Male grasshopper shows XO gametes while female grasshopper shows XX.

In birds, chromosomal composition of the egg determines the sex. This is called genetic sex determination, with females being heterogametic. But being heterogametic is alone not responsible for sex determination. Other factors like specific genes or autosomal, sex chromosome ratio, etc., are responsible.

The cross between white and red flower of Antirrhinum shows pink colour in the offspring plant i.e., neither white or red is completely dominant.

Quantitative inheritance is a type of inheritance controlled by one or more genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part or unit of the trait, the full trait being shown only when all the dominant alleles are present. The genes involved in quantitative inheritance are called polygenes. A polygene is defined as a gene where a dominant allele controls only a unit or partial quantitative expression of a trait.

According to law of segregation or law of purity of gametes, factors or alleles of a pair segregate from each other during gamete formation, such that a gamete receives only one of the two factors and do not show any blending.