Zoology Genetics - 1

In X-linked recessive inheritance, males are primarily affected because they have only one X chromosome. Affected females require an affected father and a carrier or affected mother to inherit two recessive alleles. Sons of affected females always receive the X chromosome with the recessive allele and will be affected. These traits can skip generations through carrier females, so S is false.

The equal sex ratio at birth occurs because during male meiosis, X and Y chromosomes segregate randomly into different sperm cells. Each sperm carries either X or Y, and fertilization of an X-bearing egg is equally likely with either type. Egg cells always contain X chromosomes, so they don't determine sex - the sperm does.

For a heterozygous female (X^H X^h) crossed with a normal male (X^H Y): each child has 1/4 chance of being unaffected (X^H X^H female or X^H Y male). So P (unaffected) = 3/4. For 5 offspring, the probability exactly 2 are affected is calculated using binomial distribution: C(5,2) × (1/4)^2 × (3/4)^3 = 10 × 1/16 × 27/64 ≈ 26.3%.

The Philadelphia chromosome is formed by a reciprocal translocation between chromosome 9 and chromosome 22, specifically t(9;22)(q34;q11). This creates the BCR-ABL fusion gene associated with chronic myeloid leukemia. The other chromosome pairs listed are involved in different translocations.

When two populations with different allele frequencies merge and begin random mating, Hardy-Weinberg equilibrium is achieved in just one generation. Random mating in the first generation immediately creates the genotype proportions predicted by HWE, based on the new combined allele frequencies. No additional generations are needed.

For autosomal dominant traits, every affected individual must have an affected parent (except new mutations), males and females are equally affected, and there's a 50% transmission risk to each offspring. However, both sons AND daughters of an affected male have equal 50% chance - not all daughters and no sons. This sex-specific pattern describes X-linked inheritance, not autosomal dominant.

Allele frequencies always sum to 1, and if there are two alleles, knowing one frequency gives the other. A missing allele has frequency 0. However, two populations can have identical allele frequencies but different homozygote proportions because the frequencies themselves don't determine homozygosity - you need to know whether the population is in Hardy-Weinberg equilibrium to make that calculation.

In animals, mutation takes place by change in gene structure.

During anaphase I in paracentric inversion heterozygotes, a bridge-fragment configuration forms because crossing over within the inversion creates a dicentric chromosome (forms a bridge) and an acentric fragment (lacks a centromere). Translocation heterozygotes form quadrivalents, pericentric inversions don't produce this pattern, and duplications don't create bridges.

Classical Mendelian traits are qualitative and show discontinuous variation (distinct categories like tall vs short). They are typically monogenic (controlled by one or few genes), not polygenic. Polygenic traits show continuous variation and are called quantitative or metric traits, not qualitative.

For test cross of AaBb: P (unlinked) - parental aabb is 1/4 (25%). Q (completely linked) - aabb requires no crossover, so 50% parental types. R (10 map units) - 10% recombinants, so aabb = 45% parental. S (24 map units) - 24% recombinants, so aabb = 38% parental. The further apart the genes, the more crossing over, so fewer parental aabb progeny.

People affected with Down’s syndrome have 21 trisomy (an extra chromosome number 21). So, their chromosome count is 47 (45 + XX in females and 45 + XY in males).

Natural selection reduces genetic diversity by favoring certain alleles over others. Mutation is the ultimate source of new genetic variation - it creates novel alleles that didn't exist before. Genetic drift reduces diversity (especially in small populations), and stabilizing selection specifically reduces variation by favoring intermediate phenotypes. Only mutation introduces new genetic diversity.

Hypertrichosis (hairy ears) is a Y-linked trait, meaning the gene is located on the Y chromosome. A male with this trait passes his Y chromosome to all his sons. Since the mother is normal (no Y chromosome), 100% of sons will inherit the father's Y chromosome and thus express the trait.

Hardy-Weinberg equilibrium requires five conditions: no migration, no natural selection, random mating, no mutations, and large population size. Having multiple alleles at a locus is not a requirement - the equilibrium can be maintained with just two alleles at each locus.

The genes are 12 map units apart, so the recombination frequency is 12%. Parental gametes (AB and ab) occur 88% of the time (44% each), while recombinant gametes (Ab and aB) occur 12% of the time (6% each). When AaBb is crossed with aabb, we get 440 AaBb, 440 aabb, 60 Aabb, and 60 aaBb.

The frequency of dominant allele in such population is 0.84.

Genetic code is universal that has led to the formation of food chains and food webs.

A disease in many Africans, which reduces oxygen uptake by blood and is a genetic disease is called sickle cell anemia. The erythrocytes in sickle cell anemia are sickle-shaped instead of being biconcave discs. The blood flows in the capillaries may be partially or completely blocked by the cell causing severe pain and damage to tissues.

In a three-point cross, the double crossover classes have the lowest frequency. Here, ch b+ cn+ (60) and ch+ b cn+ (111) are the double crossovers. Comparing these to the parental types (ch+ b+ cn+ at 780 and ch b cn at 769), we see that cn+ stays with ch+ (parental) and with ch (double crossover), meaning cn is in the middle. The gene order is ch-cn-b.