X-linked inheritance

Orange or black fur is a single, X-linked, co-dominant trait in cats. Calico cats have white fur with patches of orange and black. Is it true that calico cats can only be female?

• Yes, because they have 2 'X' chromosomes, and therefore can have both the orange and black alleles (X^O X^B)

• No, males can be calico because they can have both the alleles for orange and black (X^O Y^B)

• Yes because the trait is found on the X chromosome (X^OB X)

• No, males can be calico because the trait is found on the X chromosome (X^OB Y)

Answer explanation

The trait is X-linked so the allele is only found on the X chromosome. Males only have one X chromosome, so can only get one of the color alleles.

X^BY or X^OY

What percentage of the offspring will have this recessive disease?

• 0%
• 25%
• 50%
• 75%

Colorblindness is more common in males than in females because......

• fathers pass the allele for colorblindness to their sons only

• the allele for colorblindness is located on the Y chromosome

• the allele for colorblindness is recessive and located on the X chromosome

• males who are colorblind have two copies of the allele for colorblindness

Which of the following genotypes belongs in the red box in the Punnett square?

• X^BX^b
• X^bX^b
• X^bY
• X^BY

Which of the following genotypes belongs in the red box in the Punnett square, and what would the genotype be for the trait?

• X^BX^b - female heterozygous
  
• X^bX^b - female homozygous recessive
• X^BX^B  - female homozygous dominant
• X^bY - male heterozygous recessive

Duchenne muscular dystrophy is a recessive X-linked disorder.

According to the Punnett square, what percentage of offspring will have the disorder?

• 0%

• 25%

• 50%

• 100%

What percentage of the female offspring will be carriers for this recessive disease?

• 0%
• 50%
• 100%
• 25%

Males are more likely to suffer from a sex-linked disease or disorder because...

• males are the weaker sex

• males have less DNA

• males have 1 X chromosome, so the disorder is more likely to be expressed

• females are the stronger sex

What chromosome letter pairs are used to represent males and females

• XX = female and YY = male

• XX = female and XY = male

• XX = male and YY = female

• XX = male and XY = female

Colorblindness is a recessive X-linked disorder.

Which genotype represents a male with normal vision?

• X^N Y^N

• Xⁿ Yⁿ

• X^N Y

• Xⁿ Y