The Genetic Basis of Inheritance From Chromosomes to
The Genetic Basis of Inheritance From Chromosomes to Genes
Chromosomal Basis of Inheritance • Traits are transmitted by chromosomes which contain units of heredity called genes • Genes are formed from DNA
Mendel and the History of Genetics • Gregor Mendel • Born 1822 • Began his work at age 21 • A monk and school teacher interested in plant breeding • Studied pea plants • Discovered the basic principles of heredity
Mendel’s Studies • Pea plants were an ideal choice for study • Displayed seven traits in one of two contrasting forms: – seed shape, seed color, seed coat color, pod shape, pod color, flower position, stem length • Mendel studied 30, 000 plants in 7 years – Kept careful records & looked for mathematical patterns
The Experiment • P Generation: – Cross 2 pure lines – – One white, one purple • F 1 are all purple – Purple is dominant • Cross F 1 x F 1 (all purple) – ¼ F 2 offspring are white – White is recessive
Genes and Alleles • We now know that the heredity units are genes. • The separate forms of a gene that Mendel called ‘factors’ = alleles • One allele in a pair may prevent the other from being expressed • If an offspring carries 2 dominant, or one dominant and one recessive allele, the offspring will appear to have the dominant trait. • If an offspring carries 2 recessive alleles, it will appear to have the recessive trait.
Each individual carries one copy (allele) of a gene on the chromosome from their mother, and a second copy on the homologous chromosome from their father.
Hereditary Terminology • Genes – The units of heredity • Alleles – The separate forms of a gene that Mendel called factors • Genotype - The set of alleles for a characteristic possessed by an organism • Phenotype - The appearance of the organism; the trait that is actually expressed • Homozygous - The 2 alleles in a pair are identical – An individual can be either homozygous dominant (2 identical dominant alleles) or homozygous recessive (2 identical recessive alleles) • Heterozygous – The 2 alleles in a pair are different
Representing Genes & Alleles • Generally the capitalized first letter of the dominant trait is used to represent the dominant allele • The recessive allele is then lower case of the same letter (e. g. T for tall, so t for short) • Each characteristic can be represented by a pair of letters representing the genes • Traits carried on sex chromosomes are written as a superscript of the X or Y chromosome
Genotype vs. Phenotype
Punnett Squares • A way to visualize crosses • Punnett square can be used to determine probability of different genotypes or phenotypes • Each box contains a possible combination of alleles for offspring • Punnett square can be used to determine probability of different genotypes or phenotypes
Incomplete Dominance • Exhibited when the heterozygote has an intermediate phenotype between that of either homozygote. Mader: Biology 8 th Ed.
Co-Dominance • Occurs when both alleles for a gene are expressed in a heterozygote • Example – Red & white produce roan color in some animals because both genes are expressed
Multi-allele Systems • Some traits are the result of more than 2 possible alleles at a locus • The ABO blood group is an important example • 3 possible alleles: – IA - produces A antigen; anti-B antibodies in blood serum – IB - produces B antigen; anti-A antibodies in serum – i - produces no antigen; anti-A & anti-B antibodies in serum • IA & IB are co-dominant alleles • i is recessive
The ABO Blood System
Environment and Phenotype • Himalayan Rabbits - Enzyme coding for black fur is active only at low temperatures. – Black fur only occurs on extremities. Mader: Biology 8 th Ed.
Sex Linkage • Autosomal trait – a gene carried on a non-sex chromosome & present in 2 copies • Sex linked trait – a gene carried on a sex chromosome that is present in both sexes; one copy in one sex, 2 in the other – X chromosome in mammals – Z chromosome in birds (one copy in females, 2 in males) • Y- linked trait – a gene carried on the Y chromosome in humans; very rare
Showing Sex Linkage • Symbols are written as superscript of the sex chromosome: – Xa - X chromosome carrying the recessive allele – XA – X chromosome carrying the dominant allele – No superscript is used for the normal or wild type allele • Suspect sex linkage when the ratios of phenotypes are different in males and females
Pedigrees • Used to determine mode of inheritance when few individuals, but several generations are involved • Assume genetic trait discussed is rare, so individuals marrying into the family are not assumed to carry the trait • Symbols: O female shaded = affected partial shading = carrier
Pedigree Analysis
Epistasis • A gene at one locus alters expression of a second gene • Example: If black (B) is dominant over brown (b) in mice, but. . . • A second gene (C) determines if fur pigment is made at all, • Then you must be (CC) or (Cc) before the mice can be either black or brown
Diagramming Epistasis
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