Genes Chromosomes Numbers Chapter 10 2 Genes Chromosomes

Genes, Chromosomes & Numbers Chapter 10. 2

Genes & Chromosomes • Genes – Sequences of DNA that code for traits – Found on chromosomes o 1000 or more genes

Diploid Cell • Body cells (except for sperm and egg= sex cells) • Contain two of each chromosome (2 n) – one chromosome from each parent during fertilization

Haploid Cell • Gametes (sex cells)sperm and egg • Contain one of each kind of chromosome • Said to contain n number of chromosomes

Practice Questions: Humans contain 46 chromosomes in their body cells. How many chromosomes are in the gametes? 23 A fruit fly has 4 chromosomes in its gamete cells. How many chromosomes are in the diploid cell? 8

Homologous Chromosomes • Paired chromosomes – genes for the same traits in the same order • Not always identical to each other – heterozygous • different alleles for the same gene – homozygous • Contain same alleles for a trait F F A a B B

Why Meiosis? • If mitosis was only type of cell division, – gametes would have same number of chromosomes as the body cells – During fertilization, gametes would have twice the number of chromosomes as each of its parents. 46 + 46 = 92 Egg + sperm = zygote (fertilized egg)

Meiosis • produce gametes • contain half the number of chromosomes as a parent’s body cell

• 2 separate divisions: Meiosis I & Meiosis II – Begin with 1 diploid (2 n) cell. – End with 4 haploid (n) gametes 23 + 23 = 46 Egg + sperm = zygote (fertilized egg)

Sexual Reproduction • Production and fusion of haploid cells to form a diploid zygote (fertilized egg)