20. Quantitative Genetics

Heritability

Open Question
Cattle breeders would like to improve the protein content and butterfat content of milk produced by a herd of cows. Narrow sense heritability values are 0.60 for protein content and 0.80 for butterfat content. The average percentages of these traits in the herd and the percentages of the traits in cows selected for breeding are as follows.Trait Herd Average Selected CowsProtein content 20.2% 22.7%Butterfat content 6.5% 7.4%Which trait is likely to be the most responsive to artificial selection applied by the cattle breeders through selection of cows for mating?
Open Question
Cattle breeders would like to improve the protein content and butterfat content of milk produced by a herd of cows. Narrow sense heritability values are 0.60 for protein content and 0.80 for butterfat content. The average percentages of these traits in the herd and the percentages of the traits in cows selected for breeding are as follows.Trait Herd Average Selected CowsProtein content 20.2% 22.7%Butterfat content 6.5% 7.4%Determine the selection differential (S) for each trait in this herd.
Open Question
In human gestational development, abnormalities of the closure of the lower part of the mid-face can result in cleft lip, if the lip alone is affected by the closure defect, or in cleft lip and palate (the roof of the mouth), if the closure defect is more extensive. Cleft lip and cleft lip with cleft palate are multifactorial disorders that are threshold traits. A family with a history of either condition has a significantly increased chance of a recurrence of mid-face cleft disorder in comparison with families without such a history. However, the recurrence risk of a mid-face cleft disorder is higher in families with a history of cleft lip with cleft palate than in families with a history of cleft lip alone.Suppose a friend of yours who has not taken genetics asks you to explain these observations. Construct a genetic explanation for the increased recurrence risk of mid-face clefting in families that have a history of cleft disorders versus families without a history of such disorders.
Open Question
In human gestational development, abnormalities of the closure of the lower part of the mid-face can result in cleft lip, if the lip alone is affected by the closure defect, or in cleft lip and palate (the roof of the mouth), if the closure defect is more extensive. Cleft lip and cleft lip with cleft palate are multifactorial disorders that are threshold traits. A family with a history of either condition has a significantly increased chance of a recurrence of mid-face cleft disorder in comparison with families without such a history. However, the recurrence risk of a mid-face cleft disorder is higher in families with a history of cleft lip with cleft palate than in families with a history of cleft lip alone.Construct a similar explanation of why the recurrence risk of a cleft disorder is higher in families with a history of cleft lip with cleft palate than in families with a history of cleft lip alone.
Open Question
The children of couples in which one partner has blood type O (genotype ii) and the other partner has blood type AB (genotype IᴬIᴮ) are studied.What is the expected concordance rate for blood type of MZ twins in this study? Explain your answer.
Open Question
The children of couples in which one partner has blood type O (genotype ii) and the other partner has blood type AB (genotype IᴬIᴮ) are studied.What is the expected concordance rate for blood type of DZ twins in this study? Explain why this answer is different from the answer to part (a).
Open Question
Answer the following in regard to multifactorial traits in human twins.If the trait is substantially influenced by genes, would you expect the concordance rate to be higher in MZ twins or higher in DZ twins? Explain your reasoning.
Open Question
Answer the following in regard to multifactorial traits in human twins.If the trait is produced with little contribution from genetic variation, what would you expect to see if you compared the concordance rates of MZ twins versus DZ twins? Explain your reasoning.
Open Question
Floral traits in plants often play key roles in diversification, in that slight modifications of those traits, if genetically determined, may quickly lead to reproductive restrictions and evolution. Insight into genetic involvement in flower formation is often acquired through selection experiments that expose realized heritability. Lendvai and Levin (2003) conducted a series of artificial selection experiments on flower size (diameter) in Phlox drummondii. Data from their selection experiments are presented in the following table in modified form and content.

Calculate the realized heritability for each year and the overall realized heritability.
Open Question
Floral traits in plants often play key roles in diversification, in that slight modifications of those traits, if genetically determined, may quickly lead to reproductive restrictions and evolution. Insight into genetic involvement in flower formation is often acquired through selection experiments that expose realized heritability. Lendvai and Levin (2003) conducted a series of artificial selection experiments on flower size (diameter) in Phlox drummondii. Data from their selection experiments are presented in the following table in modified form and content.

Considering that differences in control values represent year-to-year differences in greenhouse conditions, calculate (in mm) the average response to selection over the three-year period.
Open Question
Floral traits in plants often play key roles in diversification, in that slight modifications of those traits, if genetically determined, may quickly lead to reproductive restrictions and evolution. Insight into genetic involvement in flower formation is often acquired through selection experiments that expose realized heritability. Lendvai and Levin (2003) conducted a series of artificial selection experiments on flower size (diameter) in Phlox drummondii. Data from their selection experiments are presented in the following table in modified form and content.

In terms of evolutionary potential, is a population with high heritability likely to be favored compared to one with a low realized heritability?
Open Question
Floral traits in plants often play key roles in diversification, in that slight modifications of those traits, if genetically determined, may quickly lead to reproductive restrictions and evolution. Insight into genetic involvement in flower formation is often acquired through selection experiments that expose realized heritability. Lendvai and Levin (2003) conducted a series of artificial selection experiments on flower size (diameter) in Phlox drummondii. Data from their selection experiments are presented in the following table in modified form and content.

In terms of evolutionary potential, is a population with high heritability likely to be favored compared to one with a low realized heritability?
Open Question
Congenital dislocation of the hip is a threshold condition in which the head of the femur (the femoral head) is out of its normal position relative to the bones that will form the socket of the hip (the acetabulum). This misplacement can lead to potentially serious orthopedic problems later in life if the condition is not treated in infancy. Numerous studies have shown that (a) brothers and sisters of infants born with congenital hip dislocation are more likely to develop the condition than are the siblings of those without the condition. These studies also find that (b) more female infants than male infants have the trait, and (c) if the affected child is a girl, the risk to her siblings is lower than if the affected infant is a boy. Explain the meaning of the three observations (a, b, and c) in the context of proposing a threshold model that explains these observations.