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Chapter 5
The Inheritance of SingleGene Differences
Alleles at single locus
24 and 26 Jan, 2005
Overview
• In matings, precise phenotypic ratios are produced in descendants
as a result of chromosome segregation.
• In heterozygotes, alleles segregate equally into meiotic products.
• Progeny ratios can be predicted from known genotypes of
parents.
• Parental genotypes can be inferred from phenotypes of progeny.
• In many organisms, sex chromosomes determine sex.
• X-linked genes can show different phenotypic ratios in male and
female progeny.
• In humans, single-gene traits can be studied in pedigrees.
• Organelle genes are inherited maternally.
Meiotic chromosome segregation
• In meiosis, each of the four haploid
products receives one of each kind of
chromosome
– A/A homozygotes  all get A chromosomes
– A/a heterozygotes  half get A chromosomes
half get a chromosomes
• As a consequence of chromosome
segregation, alleles of heterozygotes
segregate equally
Equal segregation
• First observed by Mendel in crosses with
peas
• Readily observed using some fungi and
protists in which all four haploid products
of meiocyte (tetrad) can be analyzed
A
a
A
a
Aa
Tetrads and Octads
Diploid crosses (1)
• Three possible diploid genotypes
A/A
a/a
A/a
• Six possible diploid crosses
Cross
A/A  A/A
a/a  a/a
A/A  a/a
A/a  A/A
A/a  a/a
A/a  A/a
Genotypic ratio
A/A
a/a
A/a
1A/A:1A/a
1A/a:1a/a
1A/A:2A/a:1a/a
Phenotypic ratio
all A
all a
all A
all A
1A:1a
3A:1a
Diploid crosses (2)
•Crosses between individuals heterozygous
for the same single gene are also called
monohybrid crosses
•A heterozygote for unexpressed recessive
allele is sometimes called a carrier,
particularly in humans
•A cross between an unknown genotype (e.g.,
A/–) and the homozygous recessive genotype
(a/a) is called a testcross
Mendel
Testcross
Distinguishes between A/A and A/a genotypes mated
to a/a based on phenotypes of offspring
if all progeny are dominant phenotype, then unknown is
A/A
if at least one offspring is recessive phenotype, then
unknown is A/a
If:
genotypic outcome phenotypic outcome
A/A  a/a
A/a
all A
A/a  a/a
1A/a:1a/a
1A:1a
Autosomes and sex chromosomes
• Sex chromosomes determine sex in most
animals and some plants
– usually only one pair
– one sex has two alike (e.g., XX)
– one sex has two different types (e.g., XY)
• Remaining chromosomes are called
autosomes
• X chromosomes and autosomes contain
numerous genes
• Y chromosomes typically have few genes
Sex chromosome inheritance
in humans
• 46A XX is female, homogametic (only X gametes)
• 46A XY is male, heterogametic (X and Y gametes)
• Segregate equally into gametes at meiosis
Sperm
Gametes
Eggs
50% X
50% Y
50% X
XX
XY
50% X
XX
XY
Sex-linked cross
Pedigrees (1)
•
•
•
•
Analysis of inheritance in families
Typically small number of offspring
Mendelian ratios rarely observed
Allow inferences concerning genotypes and
predictions concerning phenotypes of
offspring (genetic counseling)
unaffected
male
unaffected
female
affected
male
affected
female
Pedigrees (2)
• Two children, one of each sex, show the trait
• Conclusions:
– must be autosomal recessive trait
– parents must be heterozygous
– 2/3 chance that each unafflicted child is heterozygous
Categories of inheritance
• Autosomal recessive
– e.g., PKU, Tay-Sachs, albinism
• Autosomal dominant
– e.g., Huntington disease
• X-linked recessive
– e.g., color-blindness, hemophilia
• X-linked dominant
– e.g., hypophosphatemia
• Y-linked
• Organelle
Assignment: All Chapter 5 problems,
especially 3, 4, 7-12, 15, 17, 20-28.
Continue with bioinformatics tutorial.