Introduction to Genetics & Meiosis HONORS BIOLOGY

Genetics – study of heredity Heredity – passing on characteristics from parents to offspring Traits – inherited characteristics

Gregor Mendel 19th Century Austrian Monk First important studies of heredity

Monohybrid Crosses Two parents, differing by a single trait Parents = P1 First Generation is called F1 – Filial 1 Second Generation is F2

How information is transferred Gametes – sex cells Fertilization – uniting male and female sex cells Genetic information is located in genes, which are located on chromosomes. Each trait is represented by two alleles. Rule of Unit Factors-one factor(allele) from each parent

Rule of Dominance An allele can carry either a Dominant or a Recessive trait. For a recessive trait to be “expressed” , both alleles must carry it. A dominant trait is represented by a capital letter, recessive by a lower case.

Law of Segregation The two alleles for a trait separate when a gamete is formed. The parent passes on, at random, one allele to each off-spring.

Gene expression Phenotype – the way an organism “looks”. Genotype – the actual gene combination an organism carries. Homozygous – two alleles for a trait are the same. Heterozygous – two alleles for a trait are different.

B b BB bB Bb bb

Punnett Squares of all Possible crosses: BB x BB BB x Bb Bb x Bb Bb x bb bb x bb B BB b Bb bB bb

PART II MEIOSIS

Diploid Cell Cell that contains two of each chromosome Supports Mendel’s conclusion that organisms have two factors (alleles) for each trait One allele is on each of the paired chromosomes

Homologous Pairs Together the two chromosomes of each pair in a diploid cell help determine what the individual organism looks like. These paired chromosomes are called homologous pairs or homologous chromosomes.

Have genes for the same traits arranged in the same order, but there are different possible alleles for the same gene. Two chromosomes in a homologous pair are not identical

Haploid cells A cell with one of each kind of chromosome - gamete Male gametes are called sperm Female gametes are called eggs When a sperm fertilizes an egg the resulting off-spring is called a zygote

Meiosis Cell division which produces gametes containing half the number of chromosomes of the parent Two separate divisions - Meiosis I & Meiosis II Meiosis I begins with 1 diploid cell and Meiosis II ends with four haploid cells

Interphase Cell carries out its normal functions Sex cells replicate their chromosomes Each chromosome then consists of two identical sister chromatids held together by a centromere

Prophase I Chromosomes coil up and spindle forms Each pair of homologous chromosomes comes together top form a four-part structure called a tetrad Pair so tightly that crossing-over can occur resulting in new allele combinations

Metaphase I Tetrads line up on mid-line of spindle

Anaphase I Homologous chromosomes separate and move to opposite ends of the cell

Telophase I Spindle is broken down Chromosomes uncoil Cytoplasm divides to form two new cells each cell has only half the genetic information of the original cell because it has only one chromosome from each homologous pair

Second Phase of Meiosis The newly formed cells must divide again to create the haploid cells

Comparing Meiosis and Mitosis Chromosome behavior Mitosis: Homologous chromosomes independent Meiosis: Homologous chromosomes pair forming bivalents until anaphase I Chromosome number- reduction in meiosis mitosis- identical daughter cells meiosis- daughter cells haploid Genetic identity of progeny: Mitosis: identical daughter cells Meiosis: daughter cells have new assortment of parental chromosomes Meiosis: chromatids not identical, crossing over