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University of Toronto Scarborough
Biological Sciences
Kenneth Welch

Heredity Genetics • The study of the mechanism of heredity • Nuclei of all human cells (except gametes) contain 46 chromosomes • Sex chromosomes determine the genetic sex (XX = female, XY = male) • Karyotype – the diploid chromosomal complement displayed in homologous pairs • Genome – genetic (DNA) makeup represents two sets of genetic instructions – one maternal and the other paternal Alleles • Matched genes at the same locus on homologous chromosomes • Homozygous – two alleles controlling a single trait are the same • Heterozygous – the two alleles for a trait are different • Dominant – an allele masks or suppresses the expression of its partner • Recessive – the allele that is masked or suppressed Genotype and Phenotype • Genotype – the genetic makeup • Phenotype – the way one’s genotype is expressed Segregation and Independent Assortment • Chromosomes are randomly distributed to daughter cells • Members of the allele pair for each trait are segregated during meiosis • Alleles on different pairs of homologous chromosomes are distributed independently • The number of different types of gametes can be calculated by this formula: 2 , where n is the number of homologous pairs • In a man’s tes23s, the number of gamete types that can be produced based on independent assortment is 2 , which equals 8.5 million possibilities Crossover • Homologous chromosomes synapse in meiosis I • One chromosome segment exchanges positions with its homologous maternal counterpart • Crossing over occurs, forming a chiasma • Genetic information is exchanged between homologous chromosomes • Two recombinant chromosomes are formed Random Fertilization • A single egg is fertilized by a single sperm in a random manner • Considering independent assortment and random fertilization, an offspring represents one out of 72 trillion (8.5 million 8.5 million) zygote possibilities Dominant-Recessive Inheritance • Reflects the interaction of dominant and recessive alleles • Punnett square – diagram used to predict the probability of having a certain type of offspring with a particular genotype and phenotype • Example: probability of different offspring from mating two heterozygous parents T = tongue roller and t = cannot roll tongue • Examples of dominant disorders: achondroplasia (type of dwarfism) and Huntington’s disease • Examples of recessive conditions: albinism, cystic fibrosis, and Tay-Sachs disease • Carriers – heterozygotes who do not express a trait but can pass it own to their offspring Incomplete Dominance • Heterozygous individuals have a phenotype intermediate between homozygous dominant and homozygous recessive • Sickling is a human example when aberrant hemoglobin (Hb) is made from the recessive allele (s) SS = normal Hb is made Ss = sickle-cell trait (both aberrant and normal Hb is made) ss = sickle-cell anemia (only aberrant Hb is made) Multiple-Allele Inheritance • Genes that exhibit more than two alternate alleles • ABO blood grouping is an example • Three alleles (I , I , i) determine the ABO blood type in humans • I and I are codominant (both are expressed if present), and i is reces
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