Extensions on
Mendelian Genetics
Genotype vs. Phenotype
• Since an individual has two copies of each
gene, a heterozygous individual carries the
recessive gene without showing it.
• This is referred to as a carrier.
• Some human disorders are caused by
recessive alleles (ex: cystic fibrosis).
• Other disorders, such as Huntington’s disease,
are caused by dominant alleles.
• Which one do you think is more common?
Cystic Fibrosis
Sex-Linked Genes
• Genes located on sex chromosomes (X or Y)
are called sex-linked genes.
• Y chromosome is smaller and has fewer genes
– Mostly just for sex determination
• Females are XX; males are XY
• Males express all genes from both
chromosomes because there is not another
copy to “mask” recessive traits
Color Blindness
• Recessive trait on the X chromosome
• Females need two copies to express the trait
• Males only need one copy
Practice
• The gene for colorblindness is carried on the X
chromosome and is recessive. A colorblind
man marries a woman of normal vision. They
have a daughter who is colorblind.
– What are the genotypes of each parent?
– What is the daughter’s genotype?
– Show a Punnett square predicting the possible
genotypes and phenotypes that could be
produced if the couple has another child.
Practice
• Hemophilia is carried on the X chromosome
and is recessive to normal blood type. Cross a
carrier female to an affected man.
– What percentage of the offspring will be carrier
females?
– What percentage will be normal males?
– What percentage will be hemophiliac females?
– How many genotypes are possible among the
offspring?
Complex Inheritance Patterns
• Sometimes, phenotype results as an
interaction between both alleles
– Incomplete dominance
– Codominance
Incomplete Dominance
• Both traits are mixed in a heterozygous
offspring
• Example: four o’clock plant
– Homozygous red x homozygous white results in
pink offspring
Incomplete Dominance Practice
• In radishes, the gene that controls color
exhibits incomplete dominance. Purebreeding red radishes (RR) crossed with purebreeding white (WW) radishes make purple
radishes (RW). What are the genotypic and
phenotypic ratios when you cross a purple
radish with a white radish?
Codominance
• Both alleles are expressed completely
– Homozygous red crossed with homozygous white
would result in red and white spotted offspring
– Example: roan cows
Codominance Practice
In shorthorn cattle hair color is decided by a pair
of codominant traits. R is the allele for red hair
color and W is the allele for white hair color. A
cow with the heterozygous genotype is roan in
color, meaning its coat contains both white and
red hairs.
What would be the expected phenotypes of the
offspring if a white cow is bred with a roan cow?
Polygenic Traits
• Traits produced by more than one gene
– Skin color, eye color, blood type
– Blood type is polygenic and codominant!
Blood Type
• A man with type AB blood marries a woman
with type B blood. Her mother has type O
blood. List the expected phenotype &
genotype frequencies of their children.
Blood type Practice
1. What are the expected genotypes and phenotypes of
the offspring if the father is type A heterozygous and
the mother is type B heterozygous.
2. What are the expected genotypes and phenotypes of
the offspring if the father is type O and the mother is
type AB?
3. Ralph has type B blood and his wife Rachel has type A
blood. They are very shocked to hear that their baby
has type O blood, and think that a switch might have
been made at the hospital. Can this baby be theirs?
Explain why or why not (use a Punnett square to
help).
Pedigrees
• Pedigrees help trace phenotypes and
genotypes in a family
Pedigrees
• Pedigrees can be analyzed to help you
answer several different questions:
–Is this trait dominant or recessive?
• Recessive traits will be less common
–Is this trait autosomal or sex linked?
• Autosomal traits should affect equal
numbers of males and females
• Sex linked traits will affect more males
Is this trait dominant or recessive?
Sex-linked or autosomal?
Is this trait dominant or recessive?
Sex-linked or autosomal?
Is this trait dominant or recessive?
Sex-linked or autosomal?
Hemophilia in the Royal Family
Practice
1
3
2
• What are the genotypes
of individuals 1 and 2?
• How many of their
offspring are
colorblind?
• Are any carriers?
• What is the genotype of
individual 3? How do
you know?
Practice
Karyotyping
• A karyotype is a picture
of all the chromosomes
in a cell
• Chromosome mapping
can help diagnose
genetic disorders such
as Down syndrome
(having an extra copy of
the 21st chromosome)
or Klinefelter’s (XXY)