Lesson Objectives At the end of this lesson you should be ableto • Give a definition for a gamete • Understand gamete formation • Give the function of gamete in sexual reproduction • Define fertilisation • Define allele • Differentiate between the terms homozygous and heterozygous
Lesson Objectives (cont.) At the end of this lesson you should be ableto • Differentiate between genotype and phenotype • Differentiate between dominant and recessive • Show the inheritance to the F1 generation in a cross involving: • Homozygous parents • Heterozygous parents • Sex determination • Show the genotypes of parents, gametes and offspring
Sexual Reproduction • Involves two parents • Each parent makes reproductive cells - called gametes
Outline of Fertilisation • Gametes join together by fertilisation • Form a diploid zygote • This develops into an embryo • Eventually into a new individual • New individual resembles both parents – but is not identical to either
What are Gametes? • Reproductive Cells • Formed by meiosis • Contain single sets of chromosomes - haploid • Capable of fusion to form zygote - diploid • Zygote contains genetic information of both gametes
Human Chromosomes • We have 46 chromosomes, or 23 pairs. • 44 of them are called autosomes and are numbered 1 through 22. Chromosome 1 is the longest, 22 is the shortest. • The other 2 chromosomes are the sex chromosomes: the X chromosome and the Y chromosome. • Males have and X and a Y; females have 2 X’s: XY vs. XX.
Sex Determination The basic rule: If the Y chromosome is present, the person is male. If absent, the person is female.
Meiosis • the X and Y chromosomes separate and go into different sperm cells: • ½ the sperm carry the X and the other half carry the Y. • All eggs have one of the mother’s X chromosomes • The Y chromosome has the main sex-determining gene on it, called SRY
Sex Determination • About 4 weeks after fertilization, an embryo that contains the SRY gene develops testes, the primary male sex organ. • The testes secrete the hormone testosterone. • Testosterone signals the other cells of the embryo to develop in the male pattern.
Genetics • The study of heredity. • Gregor Mendel (1860’s) discovered the fundamental principles of genetics by breedinggarden peas.
Genetic Terms - Alleles • Alternative forms of genes. • Units that determine heritable traits. • Dominant alleles (TT - tall pea plants) a. homozygous dominant • Recessive alleles (tt- dwarf pea plants) a. homozygous recessive • Heterozygous (Tt - tall pea plants)
Phenotype • Outward appearance • Physical characteristics • Examples: 1. tall pea plant 2. dwarf pea plant
Genotype Arrangement of genes that produces the phenotype Example: 1. tall pea plant TT = tall (homozygous dominant) 2. dwarf pea plant tt = dwarf (homozygous recessive) 3. tall pea plant Tt = tall (heterozygous)
Punnett square A Punnett square is used to show the possible combinations of gametes.
T T t t Breed the P generation • tall (TT) vs. dwarf (tt) pea plants
T T produces the F1 generation Tt Tt t Tt Tt t All Tt = tall (heterozygous tall) tall (TT) vs. dwarf (tt) pea plants
T t T t Breed the F1 generation • tall (Tt) vs. tall (Tt) pea plants
T t produces the F2 generation Tt TT T 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt Tt tt t 1:2:1 genotype 3:1 phenotype tall (Tt) vs. tall (Tt) pea plants
Monohybrid Cross • A breeding experiment that tracks the inheritance of a single trait. • Mendel’s “principle of segregation” a. pairs of genes separate during gamete formation (meiosis). b. the fusion of gametes at fertilization pairs genes once again.
eye color locus B = brown eyes eye color locus b = blue eyes Homologous Chromosomes This person would have brown eyes (Bb) Paternal Maternal
B sperm B B Bb haploid (n) b b diploid (2n) b meiosis II meiosis I Meiosis - eye color
B b male gametes B Bb x Bb b female gametes Monohybrid Cross • Example: Cross between two heterozygotesfor brown eyes (Bb) BB = brown eyes Bb = brown eyes bb = blue eyes
B b 1/4 = BB - brown eyed 1/2 = Bb - brown eyed 1/4 = bb - blue eyed BB Bb B Bb x Bb b Bb bb 1:2:1 genotype 3:1 phenotype Monohybrid Cross
Dihybrid • A genetic cross where two contrasting traits are investigated • Eg: TtYy or TTYY
Law of Independent Assortment (mendels 2nd law) • When gametes are formed, each member of a pair of alleles may combine randomly with either of another pair (if genes are not linked)
The allele for tongue rolling (R) is dominant to the allele for non tongue rolling (r). Also the allele for brown hair (B) is dominant to red hair (b). Neither of these characteristics is sex linked. • Using the punnet square determine the possible F1 generation genotypes of a cross between two heterozygous parents (heterozygous for both characteristics).
In the fruit fly, Drosophila, the allele for grey body (G) is dominant to the allele for ebony body (g) and the allele for long wings (L) is dominant to the allele for vestigial wings (l). These two pairs of alleles are located on different chromosome pairs. • (i) Determine all the possible genotypes and phenotypes of the progeny of the following cross: grey body, long wings (heterozygous for both) X ebony body, vestigial wings.
R R r r Incomplete Dominance • Niether genes are dominant over the other and form an intermediate phenotype in the heterozygous offspring. • Example:snapdragons (flower) • red (RR) x white (rr) RR = red flower rr = white flower
R R produces the F1 generation Rr Rr r r Rr Rr All Rr = pink (heterozygous pink) Incomplete Dominance
Co-dominance • Both alleles are dominant and both express their phenotype (multiple alleles) in heterozygous individuals. • Example: blood 1. type A = IAIA or IAi 2. type B = IBIB or IBi 3. type AB = IAIB 4. type O = ii
IB IB IAIB IAIB IA 1/2 = IAIB 1/2 = IBi i IBi IBi Co-dominance • Example: homozygous male B (IBIB) x heterozygous female A (IAi)
Practice with Crosses http://www.zerobio.com/drag_gr11/mono.htm http://www.brooklyn.cuny.edu/bc/ahp/MGInv/MGI.Intro.html
Chromosomes and Genetics • Chromosomes are long pieces of DNA, with supporting proteins • Genes are short regions of this DNA that hold the information needed to build and maintain the body • Genes have fixed locations: each gene is in a particular place on a particular chromosome • Diploids have 2 copies of each chromosome, one from each parent. This means 2 copies of each gene.
Linkage • genes are genes located on the same chromosome, which tend to be inherited together.
Example: Drosophila fruit fly: • Genes for body colour and wing length are on the one chromosome i.e. are linked. • Grey body (G) and long wings (L) are dominant to black body (g) and vestigial wings (l). G with L and g with l. • Parents: GGLL X ggll
G G g g • L L l l • Gametes: GL X gl • G g • L l • F1: GgLl
Self-cross (if genes linked): • Parents: GgLl X GgLl • Gametes: GL gl GL gl • F2: GGLL GgLl GgLl ggll
Sex determination • Autosome: a chromosome other than the sex chromosomes. • Sex chromosomes: chromosomes that determine the sex of an individual - XX or XY.