Human Inheritance & Exceptions to the Rule of Dominance & Recessiveness.
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Transcript of Human Inheritance & Exceptions to the Rule of Dominance & Recessiveness.
Human Inheritance & Exceptions to the
Rule of Dominance & Recessiveness
Traits Controlled by Single Gene (Monohybrid)
• Many human traits are controlled by a single gene with one dominant allele and one recessive.
• Possess two very different phenotypes• Ex. Widow’s Peak or Straight Hairline
Dimples
Punnett Square for Single Gene with 2 Alleles
Multiple Alleles
• Multiple Alleles– A gene that has three or more alleles that
code for a single trait.• Human Blood Type
– Controlled by three alleles– IA, IB, i– IA and IB are codominant
Codominance
• Both dominant and recessive alleles are expressed (multiple alleles) in heterozygous individuals.
• Ex. Human Blood Type• Ex. #2 Erminette chicken feathers
– Both black and white feathers
Human Blood TypeBlood Types
Blood Type Combination of Alleles
A IAIA or IAi
B IBIB or IBi
AB IAIB
O ii
Codominance ProblemExample: homozygous male Type B (IBIB)
x heterozygous female Type A (IAi)
IAIB IAIB
IBi IBi
1/2 = IAIB
1/2 = IBi IA
IB IB
i
Another Codominance Problem
• Example:Example: male Type O (ii) x female type AB (IAIB)
IAi IBi
IAi IBi
1/2 = IAi1/2 = IBi
i
IA IB
i
CodominanceQuestion:
If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?
boy - type O (ii) X girl - type AB (IAIB)
CodominanceAnswer:
IAIB
ii
Parents:genotypes = IAi and IBiphenotypes = A and B
IB
IA i
i
Incomplete DominanceHeterozygotes Heterozygotes have an appearance somewhat in in
betweenbetween the phenotypes phenotypes of the dominant and the recessive. (Mix)
Example:Example: snapdragons (flower)snapdragons (flower)red (RR) x white (rr)
RR = red flowerRR = red flowerrr = white flowerr
r
R R
Incomplete Dominance
RrRr
RrRr
RrRr
RrRr
rr
rr
RR RR
All Rr =All Rr = pink pink(heterozygous pink(heterozygous pink))
Incomplete Dominance
Traits Controlled by Many Genes
• Large number of phenotypes– Usually indicates that the trait is controlled
by many genes– Genes act together as a group to produce
a single trait.• Example
– Height & Skin Color
Effect of the Environment
• Effects of genes are often altered by the organism’s surroundings.
• Ex. Since the late 1800’s, the average height in the U.S. has increased by almost 10 cm.
Male of Female?
• Sex Chromosomes– 23rd pair of chromosomes– XX genotype=Female, XY genotype=Male
Sex-Linked Genes
• Sex-Linked Traits– Traits (genes) located on the sex chromosomes– Many sex-linked traits carried on X chromosome– Many genes/alleles are not found on male Y chromosome– Because males have only one X chromosome, males are
more likely than females to have a sex-linked trait that is controlled by a recessive allele.
• Carrier– A person who has one recessive allele for a trait and one
dominant allele. Only females in sex-linked traits.– Does not have the trait but can pass the recessive (disease)
allele to her offspring.
Red-Green Colorblindness• Sex-linked trait• A girl who receives only one recessive allele (written
XC) will not have the trait.• A boy who receives only one recessive allele will be
colorblind.
Sex-linked Traits
Sex ChromosomesSex Chromosomes
XX chromosome - female Xy chromosome - male
fruit flyeye color
Example: Example: Eye color in fruit Eye color in fruit fliesflies
Sex-linked Trait ProblemExample: Eye color in fruit flies (red-eyed male) x (white-eyed female)
XRY x XrXr
Remember: the Y chromosome in males does not carry traits.
RR = red eyedRr = red eyedrr = white eyedXy = maleXX = female
XR Xr
XR Xr
Xr y
Xr y
Xr
XR y
Xr50% red eyed female
50% white eyed male
Pedigrees
• If you were a geneticist trying to study human inheritance patterns, how would you go about doing that?
• You couldn’t set up crosses with people as Mendel did with peas.
• Need to trace the inheritance of traits through many generations in a number of families.
• Pedigree– Tool used to trace the inheritance of traits in humans– A chart of “family tree” that tracks which members have a
particular trait.
Reading a Pedigree
A half-shaded circle or square indicates that a person is a
carrier.
A completely shaded circle or square indicates that a
person has the trait.
A circle or square that is not shaded in green indicates that a person neither has the trait nor is a carrier.
A horizontal line connecting a male and female represents
a marriage.
A vertical line and a bracket connect the parents to their
children.
A circle represents a female.
A square represents a
male.
Female Carriers
Human Inheritance Summarysuch as
such as
such as
such as
controlled byHuman traits
Single genes
Multiple alleles
Many genes
Sex-linked genes
Widow’s peak
Blood type
Height
Colorblindness
Summary Questions
• If a gene has multiple alleles, why can a person only have two of the alleles for the gene?
• Why to human traits such as height and skin color have many different phenotypes?
• Explain why red-green colorblindness is more common in males than in females.
• What is a pedigree? How are pedigrees used?• Could two people with widow’s peak have a child with
a straight hairline? Could two people with strait hairlines have a child with a widow’s peak? Explain.