Principles of Inheritance & Variation Test 4

When alleles of two contrasting characters are hybridized together, one of the character express and the other remains hidden .This is Mendel’s first law or law of dominance.

The genotype is the actual genetic makeup of an individual. In monohybrid cross the genotypic ration is 1:2:1 and phenotypic ratio is 3:1.

Punnett square is used to find out all the possible combination of male and female gametes. In monohybrid cross, a square of four is used and in dihybrid square of 16 is used.

A pair of contrasting characters of same plant is calledallelomophs. Alleles separate from each other during gamete formation and goes to different gametes.

The crossing of F1 to any one of the parents is called back cross. It is used to check the purity of individual. When F1 is crossed with recessive parent, it is called test cross.

If a plant with RRTt genotype is crossed with a plant that is rrtt 50% of the offspring will be tall with red fruit.

When two pea plants having red (dominant)c coloured flowers with unknown genotype are crossed, the 75% red and 25% white flowered plants will be produced by following the law of Mendel. This is possible only if the parents are heterozygotes. 

Monohybrid cross with incomplete dominance shows both genotypic and phenotypic ratio as same (1 : 2 : 1).

Genotypic ratio - 1(AA):2(Aa):1(aa)
Phenotypic ratio - 1(Red):2(Pink):1(White)

Independent Assortment describes how different genes independently separate from one another when reproductive cells develop. Independent assortment of genes and their corresponding traits was first observed by Gregor Mendel. Recombination occurs during meiosis and is a process that breaks and recombines pieces of DNA to produce new  combinations of genes. Recombination scrambles pieces of maternal and paternal genes, which ensures that genes assort independently from one another.

The expected Phenotypes in the F2 generation of the cross RRYY x rryy is Round-yellow; wrinkled-yellow; round-green and wrinkled-green in 9:3:3:1 similar to dihybrid results.

A person with unknown blood group under ABO grouping should be given Type O blood group as O is universal donor and can be transfused to any person.

Self-fertilized plants are pure line and trihybrid plant will form 8 different kinds of gametes. These gametes undergo independent assortment to form 64 combination of zygote.

In hexaploid wheat, the basic number of chromosome is 7 and haploid in hexaploid is 21 and as diploid will have 7x6 = 42 chromosome. Therefore, n=21 and X=7.

According to question, tallness (T) is dominant over dwarfism (t) and red colour (R) is dominant over white (r) fruit colour.

Parent Generation; P₁ : RRTt  x  rrtt

F₁ generation :

Phenotypic ratio = 1  (tall plant, red fruit)         
: 1 (dwarf plant, red fruit)
; thus, percent of tall plant with red fruit is 50%.

AB blood group of mother have two alleles IA and IB. The heterozygous B blood group of father has alleles IB and i. The possible outcome are IA IB,IAi,IBIB and IBi. That form AB, A, B and B blood group respectively.

An organism with lethal mutation may not even develop beyond zygote stage due to change in gene but all kinds of mutations are not lethal. Mutation is main source of variation essential for evolution.

Deviation from Medelism’s occurs due to co-dominance, multiple alleles, incomplete dominance in which single gene is not completely dominant over the other to produce dominant trait.

T.H.Morgan worked on the tiny fruit fly called Drosophila melanogaster. This tiny fly shows variation in eye colour and can be easily crossed with other to show the flow trait.

Chromosome theory of XY sex determination was proposed by Wilson and Stevens.

Crossing over occurs at four strand stage of panchytene of meiosis-I. Each homologous chromosome has two stands. Exchange of gene occurs during this period.

In birds male contain a pair of Z chromosome as sex chromosome besides autosomes while female contain one Z and one W chromosome.

Linkage is the physical association of two genes present on same chromosome. Genes located close to each other have high linkage percentage.

Male birds bears ZZ sex chromosome as sex determination is ZO type, in which ZZ is male and ZO is female.

Two genes p and q showing 1.3% of recombination while other two genes x and y show 37.2 percent of recombination. p and q genes are present close to each other as they show less recombination percentage.

In human beings, 23 pairs of chromosomes are present in diploid cells.
Twenty two pairs of chromosomes are equal in male and female but a pair sex chromosome is different in them. Male contain XY and female contain XX sex chromosome.