Inheritance：練習題

In a dihybrid cross between two individuals heterozygous for both genes (\(AaBb \times AaBb\)), where the genes are on different chromosomes and show independent assortment, what is the expected probability of obtaining an offspring with the genotype \(aabb\)?

In a dihybrid cross involving two genes, \(D/d\) and \(E/e\), the alleles are autosomal linked. An individual with the genotype \(De/dE\) produces gametes.

If no crossing over occurs during meiosis, which of the following gametes will be produced, and in what proportion?

In a species of plant, flower color is controlled by two genes that show epistasis. Gene 1 controls the production of a precursor molecule, and Gene 2 controls the conversion of that precursor into a purple pigment. If an individual is homozygous recessive for Gene 1 (\(aa\)), it cannot produce the precursor and remains white, regardless of Gene 2.

What is the expected phenotypic ratio from a dihybrid cross of two plants heterozygous for both genes (\(AaBb \times AaBb\))?

A researcher is studying the inheritance of feather color in birds. A cross between a black-feathered bird and a white-feathered bird produces offspring that all have blue feathers. When these blue-feathered offspring are crossed, the resulting ratio is 1 black : 2 blue : 1 white.

Which type of inheritance does this represent?

In a population that is in Hardy–Weinberg equilibrium, the frequency of a recessive allele for a particular gene is represented by \(q\). If the frequency of the dominant allele is \(0.7\), what is the expected frequency of heterozygous individuals in the next generation?

A dihybrid cross is performed between two plants heterozygous for two unlinked autosomal genes. The observed phenotypic ratio of the offspring deviates significantly from the expected \(9:3:3:1\) ratio.
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}Apart from linkage, suggest and explain a genetic phenomenon that could result in a modified phenotypic ratio in the offspring of such a cross.

In a dihybrid cross involving autosomal linkage, the observed phenotypic ratios in the offspring are often different from the expected \(9:3:3:1\) ratio.
Explain how the distance between the two gene loci on the same chromosome affects the frequency of recombinant phenotypes produced via crossing over.

In a species of plant, flower color is controlled by a single gene with two codominant alleles: R (red) and W (white). Heterozygous plants (RW) have pink flowers. A second gene, at a different locus, controls height where T (tall) is dominant over t (dwarf).

a) A pink-flowered, dwarf plant is crossed with a white-flowered, heterozygous tall plant. Draw a genetic diagram to determine the expected phenotypic ratio of the offspring.
b) Explain how independent assortment during meiosis contributes to the genetic variation seen in the gametes produced by the tall parent.

In a species of plant, the allele for purple flowers (P) is dominant over white flowers (p), and the allele for tall stems (T) is dominant over dwarf stems (t). A dihybrid cross was performed between two plants heterozygous for both traits.

(a) State the expected phenotypic ratio of the offspring if the genes assort independently. (1 point)
(b) In a trial, the observed offspring were: 280 purple/tall, 20 purple/dwarf, 18 white/tall, and 82 white/dwarf. Use a chi-squared (\(\chi^2\)) test to determine if these results deviate significantly from the expected ratio. The critical value for 3 degrees of freedom at \(p=0.05\) is 7.81. (4 points)
(c) Explain how autosomal linkage between these two genes would explain the observed results. (3 points)