24 and 26 Jan, 2005 Chapter 5 The Inheritance of Single- Gene
Differences Alleles at single locus
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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.
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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
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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 Aa A aA
a
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Tetrads and Octads
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Diploid crosses (1) Three possible diploid genotypes A/A a/aA/a
Six possible diploid crosses CrossGenotypic ratioPhenotypic ratio
A/A A/A A/Aall A a/a a/a a/aall a A/A a/a A/aall A A/a A/A
1A/A:1A/aall A A/a a/a 1A/a:1a/a1A:1a A/a A/a
1A/A:2A/a:1a/a3A:1a
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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
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Mendel
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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
outcomephenotypic outcome A/A a/a A/aall A A/a a/a
1A/a:1a/a1A:1a
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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
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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 Eggs
Gametes50% X50% Y 50% XXXXY 50% XXXXY
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Sex-linked cross
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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
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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
