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
Pleiotropy (pleion, more ) One gene, many phenotypic effects 40% of cats with white fur and blue eyes are deaf Phenylketonuria (lack of enzyme that breaks down Phe) Sickle cell anemia (misshapen RBCs at low O 2 concs)
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Effects of one gene are modified by one or several other genes Two genes affect coat color B black, b brown, E pigment deposited, e no pigment Examples: Coat color in mammals Color of wheat grains Fruit color in squash Bombay phenotype in ABO blood groups in man Epistasis One gene affects expression of another
Bombay phenotype
Quantitative characters vary in a population along a continuum or gradation Due to the presence of contributory or dominant (ABC) and non-contributory or recessive alleles (abc) Expression can be affected by environmental factors Polygenic inheritance Additive effects of two or more genes on a phenotypic character
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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) Aa Bb Cc x Aa Bb Cc (medium brown) brown
Parents are usually heterozygous for all traits (F2 generation) Interested in any of the following: # of genes affecting the character # of phenotypes present for the character probabilities for each phenotype sample genotypes Solving polygenic inheritance problems
A man is 5 feet 7 inches tall. 1. According to the chart on the right, how many dominant alleles (heads) and recessive alleles (tails) will he have?. 2. How many inches does each contributory allele add to the height? 3. List all possible combinations of alleles he can give to his child. 4. If he marries a woman who is 5 feet and 3 inches tall, what are the possible values for the height of their offspring? Example 1 Coin Situation Height O Tails and 6 Heads 6 feet 1 inch 1 Tail and 5 Heads 5 feet 11 inches 2 Tails and 4 Heads 5 feet 9 inches 3 Tails and 3 Heads 5 feet 7 inches 4 Tails and 2 Heads 5 feet 5 inches 5 Tails and 1 Head 5 feet 3 inches 6 Tails and 0 Heads 5 feet 1 inch
Coin Situation Example 1 Height O Tails and 6 Heads 6 feet 1 inch 5. Given parents that are both heterozygous (AaBbCc) for all, give the probability that the offspring would have the heights specified in the height table. 1 Tail and 5 Heads 5 feet 11 inches 2 Tails and 4 Heads 5 feet 9 inches 3 Tails and 3 Heads 5 feet 7 inches 4 Tails and 2 Heads 5 feet 5 inches 5 Tails and 1 Head 5 feet 3 inches 6 Tails and 0 Heads 5 feet 1 inch
number of possible combinations of r objects from a set of n objects r is specified # of dominant alleles n is the total number of alleles Listing or Forked Line Method Using the Combination Formula Using Pascal s Triangle
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? Example 2
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 inheritance 2. Sex-limited inheritance 3. Sex-influenced inheritance Sex-related inheritance expression of traits is affected by the sex of the individual
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 Sex-linked inheritance
Recessive alleles Color-blindness Duchenne muscular dystrophy Hemophilia Complete androgen insensitivity syndrome Dominant traits Hypophosphatemia Examples of X-linked traits
Red-green colorblindness is a recessive x- linked trait. If a man and a woman with normal color vision, but whose fathers are affected with the trait marry, what is the probability that they will give rise to a. A color-blind girl? b. A color-blind boy? c. A child with normal vision?
Transmission of genes from father to son Testis-determining factor (TDF/SRY gene) Hypertrichosis of the ears Examples of Y-linked (Holandric) 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 Male HH hen-feathered hen-feathered Hh hen-feathered hen-feathered hh hen-feathered rooster-feathered Sex-limited inheritance
Rooster feathering in chickens is a trait limited in expression only in males and determined by the autosomal recessive genotype hh. The dominant allele (H) produces hen-feathered males. All females are hen-feathered regardless of genotype. A rooster-feathered male is mated to a hen and they have twelve offspring: six males and six females. Three of the six males have rooster feathers. Determine the possible genotypes of the six female offspring.
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 Sex-influenced Genotype Female recessive Male dominant b1b1 bald bald b1b2 not bald bald b2b2 not bald not bald inheritance
Pattern baldness is a sex-influenced trait that is dominant in men and recessive in women. A heterozygous bald man marries a bald woman, what is the probability that they will give rise to a. An affected girl? b. An affected boy? c. A child who is unaffected?