Ch 2 pt 3: Mendelian Genetics

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Thinking about the Biology of Behavior Chapter 2 Evolution, Genetics, and Experience

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Fundamental Genetics Dichotomous traits – occur in one form or the other, never in combination True-breeding lines – interbred members always produce offspring with the same trait Mendel studied dichotomous traits in true-breeding lines of pea plants

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Mendel’s Experiments Crossed a line bred true for brown seeds with one bred true for white First generation offspring all had brown seeds When the first generation were bred, the result was ¾ brown and ¼ white seeds

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Mendel’s Experiments (continued) True-breeding lines White (ww) Brown (BB) Brown was the dominant trait, appearing in all of the first generation offspring (Bw)

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Mendel’s Experiments (continued) Phenotype – observable traits Genotype – traits present in the genes If the dominant trait is present in the genotype (Bw), it will be observed in the phenotype (brown seeds)

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Mendel’s Experiments (continued) 1st generation Bw 2nd generation consists of individuals that are BB, Bw, and ww BB, Bw = brown ww = white

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Mendel’s Experiments (continued) Each inherited factor is a gene Two genes that control the same trait are called alleles Homozygous – 2 identical alleles (BB, ww) Heterozygous – 2 different alleles (Bw)

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Chromosomes: Reproduction, Linkage, and Recombination Genes are located on chromosomes in the nucleus of each cell Humans have 23 pairs of chromosomes, with an allele on each chromosome Meiosis – a process of cell division that yields cells with just 23 chromosomes

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Chromosomes: Reproduction, Linkage, and Recombination Gametes, egg cells and sperm cells, are produced by meiosis When egg and sperm combine to form a fertilized egg (zygote), 23 pairs of chromosomes are again present Mitosis – a form of cell division that yields daughter cells that have 23 pairs of chromosomes

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Meiotic and Mitotic Cell Division Figure 2.17, p. 38

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Chromosomes: Reproduction, Linkage, and Recombination (continued) Meiosis leads to diversity as the 23 pairs of chromosomes are randomly sorted into the 2 gametes produced Linkage – the tendency of traits encoded on the same chromosome to be inherited together Crossing over – increases diversity, “shuffles the genetic deck”

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CrossingOver Crossing over shuffles the genetic material; it is why all the genes on a chromosome are not always inherited together

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Chromosomes: Structure and Replication (continued) Chromosomes are DNA molecules: double strands of nucleotide bases wrapped around each other A nucleotide on strand 1 always pairs with a particular nucleotide on strand 2 To replicate, the strands unwind; each nucleotide attracts its complementary base, making two DNA molecules identical to the original

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Sex Chromosomes and Sex-Linked Traits Sex chromosomes, X and Y, look different and carry different genes Female = XX Male = XY Sex-linked traits – influenced by genes on the sex chromosomes Dominant traits on the X chromosome will be seen more commonly in females, recessive ones in males