bioch11b - Otterville R
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Transcript bioch11b - Otterville R
Sex-linked Traits
• Traits (genes) located on the sex
chromosomes
• Sex chromosomes are X and Y
• XX genotype for females
• XY genotype for males
• Many sex-linked traits carried on X
chromosome
1
Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
2
Sex-linked Trait Problem
• Example: Eye color in fruit flies
•
(red-eyed male) x (white-eyed female)
XR Y
x
XrXr
• Remember: the Y chromosome in males does
not carry traits.
Xr
Xr
• RR = red eyed
• Rr = red eyed
R
X
• rr = white eyed
• XY = male
Y
• XX = female
3
Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
4
Female Carriers
5
Extensions of Mendelian analysis
Genes follow Mendel’s law of inheritance, but
differences in gene action can generate more
complex inheritance patterns for phenotypes
Single genes - dominance, codominance, incomplete
dominance, overdominance, allelic series, pleiotropy,
lethals
Multiple genes - epistasis, polygenic traits
Genes & the environment - sex-influenced traits,
environment-dependent gene expression, incomplete
penetrance
Squares are male Circles are females
Colored or filled in shapes are carries of the trait, partially colored shapes are carriers
Pedigree analysis
In humans, pedigree analysis is an important
tool for studying inherited diseases
Pedigree analysis uses family trees and
information about affected individuals to:
figure out the genetic basis of a disease
or trait from its inheritance pattern
predict the risk of disease in future
offspring in a family (genetic counseling)
Today... Pedigree analysis
How to read pedigrees
Basic patterns of inheritance
autosomal, recessive
autosomal, dominant
X-linked, recessive
X-linked, dominant (very rare)
Applying pedigree analysis - practice
Sample pedigree - cystic fibrosis
male
female
affected individuals
Autosomal recessive traits
• Trait is rare in pedigree
• Trait often skips
generations (hidden in
heterozygous carriers)
• Trait affects males and
females equally
Autosomal recessive diseases in humans
Most common ones
• Cystic fibrosis
• Sickle cell anemia
• Phenylketonuria (PKU)
• Tay-Sachs disease
For each of these, overdominance
(heterozygote superiority) has been suggested
as a factor in maintaining the disease alleles at
high frequency in some populations
Autosomal dominant pedigrees
• Trait is common in the pedigree
• Trait is found in every generation
• Affected individuals transmit the trait to ~1/2 of
their children (regardless of sex)
Autosomal dominant traits
There are few
autosomal dominant
human diseases
(why?), but some
rare traits have this
inheritance pattern
ex. achondroplasia
(a sketelal disorder
causing dwarfism)
X-linked recessive pedigrees
• Trait is rare in pedigree
• Trait skips generations
• Affected fathers DO
NOT pass to their sons,
• Males are more often
affected than females
X-linked recessive traits
ex. Hemophilia in European royalty
X-linked recessive traits
ex. Glucose-6-Phosphate Dehydrogenase deficiency
• hemolytic disorder causes jaundice in infants and
(often fatal) sensitivity to fava beans in adults
• the most common enzyme
disorder worldwide, especially
in those of Mediterranean
ancestry
• may confer malaria resistance
X-linked recessive traits
ex. Glucose-6-Phosphate-Dehydrogenase deficiency
X-linked dominant pedigrees
• Trait is common in pedigree
• Affected fathers pass to ALL of their daughters
• Males and females are equally likely to be affected
X-linked dominant diseases
• X-linked dominant diseases are extremely unusual
• Often, they are lethal (before birth) in males and
only seen in females
ex. incontinentia pigmenti (skin lesions)
ex. X-linked rickets (bone lesions)
Pedigree Analysis in real life: complications
Incomplete Penetrance of autosomal dominant traits
=> not everyone with genotype expresses trait at all
Ex. Breast cancer genes BRCA-1 and BRCA-2
& many “genetic tendencies” for human diseases
Pedigree Analysis in real life: complications
Sex-limited expression
=> trait only found in males OR females
Pedigree Analysis in real life
Remember:
• dominant traits may be rare in population
• recessive traits may be common in population
• alleles may come into the pedigree from 2 sources
• mutation happens
• often traits are more complex
• affected by environment & other genes
What is the pattern of inheritance?
What are IV-2’s odds of being a carrier?
Sample pedigree - cystic fibrosis
What can we say about
I-1 and I-2?
What can we say about
II-4 and II-5?
What are the odds that
III-5 is a carrier?
What can we say about
gene frequency?
What is the inheritance pattern?
What is the genotype of III-1, III-2, and II-3?
What are the odds that IV-5 would have an affected son?
III-1 has 12 kids with an unaffected wife
8 sons - 1 affected
4 daughters - 2 affected
Does he have reason to be concerned about paternity?
Breeding the perfect Black Lab
How do we get a true-breeding line for both traits??
black individuals = fetch well
grey individuals = don’t drool