Single-Gene Inheritance (Learning Objectives) Review the presence of homologous chromosomes in diploid organisms that reproduce sexually, the definitions of karyotype, autosomes and sex chromosomes. Recognize Mendel s contribution to genetics and the terminology he used. Understand and define: characteristic, trait, true-breeder, genotype, phenotype, allele, autosomal dominant and recessive traits, and a monohybrid cross. Explain Mendel s law of allele segregation. Learn what is meant by a test cross and when it is used. Explain Mendel s law of independent assortment for the simultaneous inheritance or two characters. Understand and use the Punnett square for determining genotypes and phenotypes and probability of offspring for autosomal dominant or recessive traits. Learn how pedigrees are used to determine the pattern of inheritance and make genetic predictions. Understand the significance of dominance and recessiveness of traits for Medical Genetics
Patterns of Inheritance Gregor Mendel - Priest studied variation in plants, patterns of inheritance in garden peas - Described the units of inheritance and the laws of inheritance, how genes pass from generation to generation - Used math to explain biological phenomena
Gregor Mendel Experimented from 1857-1863 Used: - Controlled plant breeding - Careful recordkeeping - Large numbers - Statistics
Terminology Character or characteristic: a heritable feature e.g. flower color Trait: variant of the character e.g. purple or white Mendel focused on characters with two variants either-or traits
Mendel had control over which plants he crossed
Mendel Studied Transmission of Seven Traits in the Pea Plant Figure 4.2 Figure 4.3
True-Breeding Plants Offspring have the same trait as parent Examples: - Round-seeded parents produce all round-seeded offspring - Yellow-seeded parents produce all yellow-seeded offspring - Short parents produce all short offspring
Mendel started with True-breeding plants F 1 generation F 2 generation F 2 ratio Purple flower- dominant trait White flower- recessive trait
Mendel's Data Mendel was looking for a model that can account for the 3:1 ratio that he observed in the F 2 generation
Mendel s Model 1. An organism inherits two alleles (one from each parent). 2. One allele is dominant and the other is recessive 3. Mendel s two laws: - Law of Segregation - Law of Independent Assortment
Mendel's First Law Segregation Figure 4.5
A Punnett square - Represents particular genes in gametes and how they may combine in offspring. - Predicts the results of a genetic cross between individuals of known genotype.
Vocabulary used in Genetics An organism with two identical alleles is homozygous for that character. Organisms with two different alleles for a character is heterozygous for that character. A description of its genetic makeup is its genotype. A description of an organism s trait is its phenotype.
Used to determine the genotype of a dominant trait Test Cross
Eye Color Wild-type human eye color is brown - Blue and green eyes stemmed from mutations or SNPs that persisted The surface of the back of the iris contributes to the intensity of eye color Figure 4.8
Inheritance of Some Common Traits Box, Figure 1 Reading 4.1, Figure 1
Other Common Mendelian Traits
Mendel's Second Law Independent Assortment If the two pairs of alleles segregate independently of each other Gametes: P generation YR and yr F1 generation YR, Yr, yr, and yr These combinations produce four distinct phenotypes in a 9:3:3:1 Figure ratio. 4.9 Figure 4.12
Monohybrid cross- inheritance of one character with one pair of alleles Dihybrid cross- inheritance of 2 characters with two pairs of alleles Crossing true-breeding plant that have yellow, round seeds (YYRR) with truebreeding plants that have green, wrinkled seeds (yyrr).
Mendel's Second Law Independent Assortment Figure 4.11 Figure 4.9
Pedigrees Symbolic representations of family relationships and the transmission of inherited traits Help in understand the past and predicting the future.
Figure 4.15 Pedigree Analysis
Autosomal Recessive Trait Albinism = Deficiency in melanin production Figure 4.17
Autosomal Dominant Trait Polydactyly = Extra fingers and/or toes Figure 4.16b
Medical Genetics: Dominance and Recessiveness Reflect the characteristics or abundance of a protein Recessive traits have loss of function Dominant traits have gain of function Recessive disorders tend to be more severe For many autosomal dominant traits, affected individuals are heterozygous (Aa) - The homozygous dominant phenotype (AA) is either lethal or very rare
Autosomal Dominant Traits
Autosomal Recessive Traits 5. More likely to occur in families with consanguinity