GENETIC INHERITANCE

Lesson Objectives At the end of this lesson you should be able to   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 able to   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

Sex Determination

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.

Male Karyotype

Female Karyotype

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 breeding garden 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

Arrangement of genes that produces the phenotype 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)

A Punnett square is used to show the possible combinations of gametes.

Breed the P generation tall (TT) vs. dwarf (tt) pea plants T t

tall (TT) vs. dwarf (tt) pea plants All Tt = tall (heterozygous tall) produces the F1 generation Tt

Breed the F1 generation tall (Tt) vs. tall (Tt) pea plants T t T t

tall (Tt) vs. tall (Tt) pea plants produces the F2 generation 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt 1:2:1 genotype 3:1 phenotype TT Tt tt

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.

Homologous Chromosomes eye color locus B = brown eyes eye color locus b = blue eyes This person would have brown eyes (Bb) Paternal Maternal

Meiosis - eye color B B Bb b b sperm haploid (n) diploid (2n) meiosis II B b meiosis I Bb diploid (2n)

Monohybrid Cross Example: Cross between two heterozygotes for brown eyes (Bb) BB = brown eyes Bb = brown eyes bb = blue eyes B b Bb x Bb male gametes female gametes

Monohybrid Cross B b Bb x Bb 1/4 = BB - brown eyed 1/4 = bb - blue eyed 1:2:1 genotype 3:1 phenotype BB Bb bb

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.

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

Incomplete Dominance R produces the Rr F1 generation r All Rr = pink (heterozygous pink) produces the F1 generation Rr

Pink Flowers?

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

Co-dominance Example: homozygous male B (IBIB) x heterozygous female A (IAi) IA IB i IAIB IBi 1/2 = IAIB 1/2 = IBi

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.    

Parents: Mother X Father X X X Y Gametes: X (Egg) X orY (Sperm)   F1 genotype: XX XY Phenotype: Female Male

The male thus determines the sex of an offspring. Mother gives an X to everyone but father gives an X or Y chromosome. There is a 50:50 chance that any child will be male/female)

In man sex-linked genes (i. e In man sex-linked genes (i.e. those on the X chromosome with no corresponding part on the Y chromosome) include those governing red-green colour blindness, muscular dystrophy and haemophilia (inability to clot blood). Females with both recessive genes for haemophilia do not survive beyond the first four months of gestation period.

Parents: Female carrier X Male normal   XHXh XHY Gametes: XH Xh XH Y

F1 XHXH XHY XHXh XhY Female Male Female Male Normal Normal Carrier Haemophili   25% chance of producing a haemophiliac child 50% chance of producing a haemophiliac son. It is the mother that determines if the son is haemophiliac or not since the father always passes the Y chromosome to his son.

Colour blindness is caused by a recessive gene on the X chromosome.   Parents: Female carrier X Male colour blind

End