) When predicting the phenotype of the offspring from a dihybrid cross, why is it so important that each gene is located in a different chromosome? B) You crossed a tall plant that also produces purple flowers with a short plant that produces purple flowers. Assume tall and purple flower are dominant traits. The offspring were: 100% tall plants, ¾ of the plants produced purple flowers and the remaining 1/4 of the plants produced white flowers. What were the possible genotypes of the parents?

a) In a dihybrid cross, if the genes are present on the same chromosome, they will not exhibit independent assortment. The genes present on the same chromosome are called linked genes. Linked genes are inherited together. The absence of independent assortment of a linked gene gives more of parental types in F2 progeny. Therefore, if genes are not present on a separate chromosome, Mendel’s ratios deviate.

b) Alleles for tallness and purple flower are dominant. In the progeny, all the plants are tall. This means that the dominant tall plant is homozygous. With respect to flower color, the trait exhibits segregation of dominant and recessive alleles and both purple and white flower progeny are obtained. This makes both the plants heterozygous for the flower color gene.

The genotype of tall and purple flower parent= TTPp

The genotype of short and purple flower parent: ttPp

luisvaschetto luisvaschetto · Answer 2 · 2021-10-23T23:39:00+02:00

Linked genes do not follow the law of independent assortment. Moreover, the genotypes of the parents of this cross are parent 1: TTPp and parent 2: ttPp.

When two genes are physically linked they are found on the same chromosome, thereby their alleles cannot segregate independently during meiosis (i.e., gamete formation).

In this case, it is imperative to indicate the alleles of each genotype:

Dominant plant height: T allele (tall plant)

Recessive plant height: t allele (short plant)

Dominant plant flower color: P allele (purple flowers)

Recessive plant flower color: p allele (white flowers)

The gametes of the parents are:

Parent 1 (tall plant that produces purple flowers): TTPp >>>> Gametes = TP; Tp

Parent 2 (short plant that produces purple flowers): ttPp >>>> Gametes = tP; tp

The genotypes F1 generation can be obtained by using the Punnet square:

TP Tp

tP 1/4 TtPP 1/4 TtPp

tp 1/4 TtPp 1/4 Ttpp

In consequence, the F1 phenotype of this cross is 100% tall plants (genotype Tt), whereas 3/4 of the plants produce purple flowers (genotype P_) and the remaining 1/4 of the plants produce white flowers (genotype pp).

In conclusion, linked genes do not follow the law of independent assortment. Moreover, the genotypes of the parents of this cross are parent 1 (tall plant that produces purple flowers): TTPp and parent 2 (short plant that produces purple flowers): ttPp.

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