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Non-Mendelian Genetics. Complete dominance Law of segregation Law of independent assortment One gene one trait. Mendelian Genetics. Codominance Incomplete dominance Multiple alleles Pleiotropy Epistasis Polygenic inheritance Sex-related inheritance. Non-mendelian Genetics. - PowerPoint PPT Presentation
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Non-Mendelian
Genetics
Mendelian Genetics
•Complete dominance•Law of segregation•Law of independent assortment
•One gene one trait
Non-mendelian Genetics
• Codominance• Incomplete dominance• Multiple alleles• Pleiotropy• Epistasis• Polygenic inheritance• Sex-related inheritance
Pleiotropy (pleion, “more”)• One gene, many
phenotypic effects
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EpistasisOne gene affects expression of another
• Effects of one gene are modified by one or several other genes
• Examples:– Coat color in mammals– Color of wheat grains– Fruit color in squash– Bombay phenotype in ABO
blood groups in man
Two genes affect coat color
B black, b brown, E pigment deposited, e no pigment
Bombay phenotype
Polygenic inheritanceAdditive effects of two or more genes on a phenotypic character
• Quantitative characters vary in a population along a continuum or gradation
• Due to the presence of contributory or dominant (ABC) and non-contributory or non-contributory or recessive alleles (abc)recessive alleles (abc)
• Expression can be affected by environmental factors
Example:– Skin
pigmentation controlled by at least three separately inherited genes
– Each dark skin allele contributes a unit of ‘darkness’ to the individual
– AABBCC very dark
– aabbcc very light
(very light) (very dark)
(medium brown)
Aa Bb Cc x Aa Bb Cc
brown
Polygenic Inheritance Problem Solving
Petal length of a plant ranges from 4mm to 12mm to 20mm. Out of 770 plants, only 3 of them have 4mm petals.
1. How many genes affect petal length?
2. How many phenotypes are present for petal length?
3. Give one genotype for a plant with 12 mm petals.
4. Give two possible genotypes for plants with 6 mm petals
5. What proportion of plants have 14 mm petals?
6. What is the phenotype of plants with 7 contributory alleles?
7. How many contributing alleles does a plant with 8 mm petals have?
Sex-related inheritanceexpression of traits is affected by the sex of the individual
• sex is an inherited phenotypic character determined by the presence/absence of certain chromosomes
• SRY (sex-determining region of Y) in humans
1. Sex-linked inheritance2. Sex-limited inheritance3. Sex-influenced
inheritance
Sex-linked inheritance
• Gene linkage – genes located on the same chromosome are inherited together
• Sex-linkage• Sex chromosomes contain genes for many characters unrelated to sex• X-linked/Y-linked gene
Examples of X-linked traits
Recessive alleles• Color-blindness• Duchenne muscular
dystrophy• Hemophilia• Testicular
feminization
Dominant traits• Hypophosphatem
ia
Examples of Y-linked (Holandric) Inheritance
Transmission of genes from father to son• Testis-determining factor (TDF/SRY
gene)• Hypertrichosis of the ears
Sex-limited inheritance
• involves autosomal genes that are expressed only in either males or females
• resulting in a part or function of the body that is present in one sex but not the other
• Examples:• milk production• cryptorchidism• feathers in domestic fowl
Genotype Female MaleHH hen-feathered hen-feathered
Hh hen-feathered hen-feathered
hh hen-feathered rooster-feathered
Sex-influenced inheritance
• dominant in one sex but recessive in the other
• autosomal
• difference in expression due to the hormonal difference between the sexes • in heterozygotes, the expression of the trait is
affected by sex hormones• homozygotes are unaffected and express the trait
regardless of the hormone produced
Sex-influenced inheritance
•pattern baldness• baldness allele is dominant in males but recessive in females
• a heterozygous male is bald, but a heterozygous female is not
Genotype Femalerecessive
Maledominant
b1b1 bald bald
b1b2 not bald bald
b2b2 not bald not bald
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