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Dihybrid croses with incomplete dominance

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In cattle, polled is dominant to horned, and roan is the result of the heterozygous condition of the genes for red and white. What proportion of the offspring of a red, heterozygous polled bull and a roan, horned cow would be expected to be roan and horned?



1. In humans color blindness is due to a sex-linked recessive allele. Two persons with normal vision produce a color-blind child. What are the genotypes of the parents? What is the sex of the child?

Answer - Father is XCY, mother is XCXc. Boy child.

2. If a trait is always transmitted directly from a father to all his sons and from those sons to all their sons, etc., on what chromosome would the gene for the character likely be?

Answer - Y

3. A phenotypically normal couple produce a son with hemophilia, a sex-linked recessive disorder. What percentage of their offspring would be expected to be carriers for hemophilia?

Father is XHY mother is XHXh. 25% of children would be carriers.

4. In cats the genotype BB is black. Bb is tortoise shell, and bb is yellow. The gene is on the X-chromosome. A tortoise-shell female is crossed with a yellow male. What offspring would be expected? Would you expect to find any tortoise-shell males?

25% XBXb tortoise shell, 25% XbXb yellow, 25% XBY black, and 25%XbY yellow. No.


1. Occasionally a tortoise-shell male cat is found, and when found is sterile. Suggest an explanation for the occurrence of tortoise-shell male cats.

Due to no disjunction. Male tortoise shell cat would be and XBXbY male.

2. Differentiate between the meiotic products expected from no disjunction in meiosis and meiosis I and meiosis II.

If nondisjunction occurs in meiosis I, all products will be effected and contain either too many or too few chromosomes. If no disjunction occurs in meiosis II, half of the products will be normal and half will be abnormal.


1. Give the possible blood groups of the offspring if both parents are of group B.

Answer - B and O

2. Give the possible blood groups of the offspring when the parents are of the following blood groups: (1) A X B, (2) B X O

Answer - in 1 all blood types are possible, in 2 only B and O are possible.

3. A homozygous gray rabbit is crossed to a heterozygous Himalayan rabbit. If both F1 females were crossed (separately) to chinchilla males, what would be the expected COMBINED phenotypic and genotypic outcome?

Answer - F1 females could be Gh (gray). A cross of Gh x ss would give 50% Gs (gray) and 50% hs (light gray). A cross of Gg x ss would give 50% Gs (gray) and 50% gs (light gray).

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Solutions with explanations and resources to answer several test crosses.

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