Question

Select from the terms below to answer questions #17-22. One term is not used.

R) Genomic imprinting

S) Methylation

T) Sex-influenced inheritance

U) Maternal inheritance

V) Sex-limited inheritance

W) Complementation

X) X-linked inheritance

15. Pattern baldness is controlled by two alleles: B, b. Females are bald if they are homozygous (BB) but males only need one allele B to be bald. This trait is an example of _________.

16. A mother passes on her mitochondrial genes to all of her children, and a father passes his mitochondrial genes to none of his children. This is an example of _________.

17. For some genetic disorders, the expression of the phenotype depends on whether an allele is inherited from the mother or father. An example in mice occurs with a gene for insulin-like growth factor II (Igf2): a heterozygous mouse will be dwarf if its normal allele comes from his mother and its mutant allele comes from his father. This is an example of _________.

18. In chickens, a single pair of alleles controls the feathering pattern of their neck and tail. Male chickens (roosters) may show cock or hen feathering; females (hens) only show hen feathering, never cock feathering. This is a case of _________.

19. Two recessive mutations that cause a “wingless” mutation in Drosophila are independently isolated. A researcher wonders whether the mutations are in the same gene or different genes, so he crosses the two strains. The outcome is that all offspring develop normal wings. Restoration of the wild-type phenotype is due to _________.

20. It has been discovered that some hereditary traits result from changes in the pattern of gene expression without alterations to the DNA sequence. A major mechanism for silencing gene expression is _________.

Answer 1

1. Pattern baldness is controlled by two alleles: B, b. Females are bald if they are homozygous (BB) but males only need one allele B to be bald. This trait is an example of T) Sex-influenced inheritance.
   
   Sex-influenced traits are those that are expressed in males and females with different genotypes. In females, it may may take two alleles of the trait for it to be expressed, while males will need only one. This difference is due to the hormonal differences between the two.
   
2. A mother passes on her mitochondrial genes to all of her children, and a father passes his mitochondrial genes to none of his children. This is an example of U) Maternal inheritance.
   
   Maternal inheritance is the extra-nuclear genetic information and the cytosolic information inherited by the progeny. This affects the final phenotype of the progeny.

3. For some genetic disorders, the expression of the phenotype depends on whether an allele is inherited from the mother or father. An example in mice occurs with a gene for insulin-like growth factor II (Igf2): a heterozygous mouse will be dwarf if its normal allele comes from his mother and its mutant allele comes from his father. This is an example of R) Genomic imprinting.
   
   Genomic imprinting occurs when for a locus, either the mother's or the father's gamete carry silencing information. This silencing leads to a pattern of inheritance where only one parent's genotype is expressed.

4. In chickens, a single pair of alleles control the feathering pattern of their neck and tail. Male chickens (roosters) may show cock or hen feathering; females (hens) only show hen feathering, never cock feathering. This is a case of V) Sex-limited inheritance.
   
   Some traits are only expressed in one gender, regardless of genotype. Such an inheritance pattern is sex-limited inheritance.

5. Two recessive mutations that cause a “wingless” mutation in Drosophila are independently isolated. A researcher wonders whether the mutations are in the same gene or different genes, so he crosses the two strains. The outcome is that all offspring develop normal wings. Restoration of the wild-type phenotype is due to W) Complementation.
   
   Complementation works when two or more genes, all of which are mutated, produce a wild-type phenotype. This may be due to a variety of factors, some of which are biochemical and others are to do with gene regulation. Overall, complementation allows double-mutants to express wild-type phenotypes.

6. It has been discovered that some hereditary traits result from changes in the pattern of gene expression without alterations to the DNA sequence. A major mechanism for silencing gene expression is S) Methylation.
   
   Methylation, in particular that of Cytosine residues and Histone proteins, is an inheritable means of controlling gene expression. Methylated DNA/Histone acts as a transcription repression signal, preventing transcription from that locus.