Genetics: Ch. 6

Autosomal dominant trait:

1.) usually appears in both sexes with equal frequency 2.) both sexes transmit the trait to their offspring 3.) does not skip generations 4.) affected offspring must have an affected parent unless they possess a new mutation 5.) When one parent is affected (heterozygous) and the other parent is unaffected, approximately half of the offspring will be affected 6.) Unaffected parents do not transmit the trait

X-linked recessive trait:

1.) usually more males than females are affected 2.) affected sons are usually born to unaffected mothers; thus, the trait skips generations 3.) approximately half of a carrier (heterozygous) mother's sons are affected 4.) never passed from father to son 5.) all daughters of affected fathers are carriers 6.) all sons of affected mothers will be affected 7.) all daughters of carrier mothers will be normal, but half will be carriers

What features of a pedigree would distinguish between a Y-linked trait and a trait that is rare, autosomal dominant, and sex-limited to males?

If the trait were Y-linked, an affected male would pass it on to all his sons, whereas, if the trait were autosomal and sex-limited, affected heterozygous males would pass it on to only half of their sons on average.

Explain how a comparison of concordance in monozygotic and dizygotic twins can be used to determine the extent to which the expression of a trait is influenced by genes or by environmental factors.

Monozygotic twins have 100% genetic identity, whereas dizygotic twins have 50% genetic identity. Any trait that is completely genetically determined will therefore be 100% concordant in monozygotic twins and 50% concordant in dizygotic twins. Conversely, any trait that is completely environmentally determined will have the same degree of concordance in monozygotic twins than in dizygotic twins, the trait is genetically influenced. Environmental influences will reduce the concordance in monozygotic twins below 100%.

What are the two types of twins and how do they arise?

Monozygotic, and dizygotic. Monozygotic twins arise when a single fertilized egg splits into two embryos in early embryonic cleavage divisions. They are genetically identical. Dizygotic twins arise from two different eggs fertilized at the same time by two different sperm. They share, on the average, 50% of the same genes.

How does the pedigree of an autosomal recessive trait differ from the pedigree of an X-linked recessive trait?

Pedigrees of autosomal recessive traits will have equal frequencies of affected male and females, whereas pedigrees of X-linked recessive traits will show mostly affected males. Also, both parents must be carriers to have children with autosomal recessive traits, whereas a mother carrying an X-linked trait can have affected sons regardless of the genotype of the father. Finally an X-linked trait is never passed from the father to his sons

For the following mode of inheritance, describe the features that will be exhibited in a pedigree in which the trait is present: X-linked dominant

X-linked dominant traits will affect males and females and will be passed from an affected male to all his daughters, but not to his sons. An affected woman (usually heterozygous for a rare dominant trait) will pass on the trait equally to half her daughters and half her sons

For the following mode of inheritance, describe the features that will be exhibited in a pedigree in which the trait is present: X-linked recessive

X-linked recessive traits will affect males predominantly and will be passed from an affected male through his unaffected daughter to his grandson. X-linked recessive traits are not passed from father to son. Any affected female must have an affected father

For the following mode of inheritance, describe the features that will be exhibited in a pedigree in which the trait is present: Y-linked inheritance

Y-linked traits will show up exclusively in males, passed from father to all of his sons

pedigree

a pictorial representation of a family history

Autosomal recessive traits often appear in pedigrees in which there have been consanguine matings, because these traits...

appear only when both parents carry a copy of the gene for the trait, which is more likely when the parents are related

How much of their genes do dizygotic and monozygotic twins have in common?

dizygotic- ~50%, same as any pair of siblings monozygotic-100% (except for rare somatic mutations)

A trait that exhibits 100% concordance for both monozygotic and dizygotic twins. What conclusion can you draw about the role of genetic factors in determining difference in the trait?

genetic factors are unimportant

monozygotic

identical twins, result when a single egg, fertilized by a single sperm, splits early in development into two separate embryos

How can you distinguish between an autosomal recessive trait with higher penetrance in males and an X-linked recessive trait?

if X-linked recessive, the trait will not be passed from father to son

When might you see an autosomal dominant trait skip generations?

it might skip generations when a new mutation arises or the trait has reduced penetrance

consanguinity

mating between closely related people

dizygotic

nonidentical twins, arise from two separate eggs, are fertilized by two different sperm, produce genetically distinct zygotes

discordance

only one member of the pair of twins has the trait

proband

person from whom the pedigree is initiated

concordance

the percentage of twin paris that are concordant for a trait

assumption for rare traits:

unaffected people entering into a family pedigree (by marriage) are considered homozygous normal

X-linked dominant trait:

1.) Both males and females are usually affected; often more females than males are affected 2.) Does not skip generations. Affected sons must have an affected mother; affected daughters must have either an affected mother or an affected father 3.) affected fathers will pass the trait on to all their daughters and none of their sons 4.) affected mothers (if heterozygous) will pass the trait on to half of their sons and half of their daughters

Mitochondrial Inheritance:

1.) both males and females are affected 2.) the trait is passed from an affected mother to all her progeny 3.) affected males do not transmit the trait to any of their progeny

What three factors complicate the task of studying the inheritance of human characteristics?

1.) mating cannot be controlled. It is not ethical or feasible to set up controlled mating experiments 2.) humans have a long generation time, so it takes a long time to track inheritance of traits over more than one generation 3.) the number of progeny per mating is limited, so phenotypic ratios are uncertain

Y-linked trait:

1.) only males are affected 2.) passed from father to sons 3.) does not skip generations

Four attribute of pedigrees:

1.) pedigrees provide concise and accurate records of families 2.) pedigrees are helpful in following and diagnosing heritable traits (ex. diseases, and medical records) by describing patterns of inheritance 3.) pedigrees are useful in mapping (locating and isolating) gens "responsible" for certain traits 4.) rule out but not necessarily prove a certain pattern of inheritance

Autosomal recessive trait:

1.) usually appear in both sexes with equal frequency 2.) tends to skip generations 3.) affected offspring are usually born to unaffected parents 4.) when both parents are heterozygous, approximately one-fourth of the offspring will be affected 5.) appears more frequently among the children of consanguine marriages

Other than the fact that a Y-linked trait appears only in males, how does the pedigree of a Y-linked trait differ from the pedigree of an autosomal dominant trait?

A Y-linked dominant trait is passed from a father to all sons, whereas an autosomal dominant trait would be passed to only half of his sons

For the following mode of inheritance, describe the features that will be exhibited in a pedigree in which the trait is present: autosomal dominant

Autosomal dominant traits will show affected males and females arising with equal frequency from a single affected parent. The trait does not usually skip generations. If the trait is fully penetrant, there will never be affected individuals resulting from two unaffected parents; however, matings between two affected parents can result in unaffected offspring

For the following mode of inheritance, describe the features that will be exhibited in a pedigree in which the trait is present: autosomal recessive

Autosomal recessive traits will show affected males and females arising with equal frequency from unaffected parents. The trait often appears to skip generations. Unaffected people with an affected parent will be carriers. Matings between two affected individuals will result in 100% affected offspring. if the genetic condition is rare, affected individuals will often result from consanguinous matings.

Who is the proband in a pedigree? Is the proband always found in the last generation of the pedigree? Why or why not?

The proband is the person of interest for whom the pedigree chart has been drawn. The proband is not necessarily found in the last generation because the proband's children, or the children of the progand's siblings often provide information about the genotype of the proband