Chapter 11 Mendel & The Gene Idea

Mendelian genetics Character Trait True-bred Hybridization heritable feature, i.e., fur color Trait variant for a character, i.e., brown True-bred all offspring of same variety Hybridization crossing of 2 different true-breds P generation parents F1 generation first filial generation F2 generation second filial generation

The Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype. When an organism has two different alleles for a trait, the allele that is expressed, overshadowing the expression of the other allele, is said to be dominant. The gene whose expression is overshadowed is said to be recessive. Mendel’s Law of Dominance

Leading to the Law of Segregation Alternative versions of genes (alleles) account for variations in inherited characteristics For each character, an organism inherits 2 alleles, one from each parent If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance The alleles for each character segregate (separate) during gamete production (meiosis). Mendel’s Law of Segregation

The Law of Independent Assortment Law of Segregation involves 1 character. What about 2 (or more) characters? Monohybrid cross vs. dihybrid cross The two pairs of alleles segregate independently of each other. Mendel’s Law of Independent Assortment

Genetic vocabulary……. Punnett square: predicts the results of a genetic cross between individuals of known genotype Homozygous: pair of identical alleles for a character Heterozygous: two different alleles for a gene Phenotype: an organism’s traits Genotype: an organism’s genetic makeup Testcross: breeding of a recessive homozygote X dominate phenotype (but unknown genotype)

Alternative Inheritance Patterns Incomplete dominance: appearance between the phenotypes of the 2 parents. Ex: snapdragons Codominance: two alleles affect the phenotype in separate, distinguishable ways. Ex: Tay-Sachs disease Multiple alleles: more than 2 possible alleles for a gene. Ex: human blood types

Alternative Inheritance Patterns Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height

Human disorders The family pedigree Recessive disorders: Cystic fibrosis Tay-Sachs Sickle-cell Dominant disorders: Huntington’s Testing: Amniocentesis chorionic villus sampling (CVS)

Chapter 12 The Chromosomal Basis of Inheritance

Chromosomal Theory of Inheritance Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment

Chromosomal Linkage Morgan Drosophilia melanogaster XX (female) vs. XY (male) Sex-linkage: genes located on a sex chromosome Linked genes: genes located on the same chromosome that tend to be inherited together

Genetic Recombination Crossing over Genes that DO NOT assort independently of each other Genetic maps The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency Linkage maps Genetic map based on recombination frequencies

Human Sex-Linkage SRY gene: gene on Y chromosome that triggers the development of testes Fathers= pass X-linked alleles to all daughters only (but not to sons) Mothers= pass X-linked alleles to both sons & daughters Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats)

Chromosomal Errors Nondisjunction: Aneuploidy: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II Aneuploidy: chromosome number is abnormal Monosomy – missing chromosome Trisomy - extra chromosome (ex. Trisomy 21 Down Syndrome) Polyploidy - extra sets of chromosomes

Chromosomal Errors Alterations of chromosomal structure: Deletion: removal of a chromosomal segment Duplication: repeats a chromosomal segment Inversion: segment reversal in a chromosome Translocation: movement of a chromosomal segment to another

Genomic Imprinting A parental effect on gene expression Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm. Fragile X syndrome: higher prevalence of disorder and mental disability in males