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GENETIC INHERITANCE Lesson Objectives At the end of this lesson you should be able to 1. Give a definition for a gamete 2. Understand gamete formation 3. Give the function of gamete in sexual reproduction 4. Define fertilisation 5. Define allele 6. Differentiate between the terms homozygous and heterozygous Lesson Objectives (cont.) At the end of this lesson you should be able to 6. Differentiate between genotype and phenotype 7. Differentiate between dominant and recessive 8. 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 Genetics • The study of heredity. • Gregor Mendel (1860’s) discovered the fundamental principles of genetics by breeding garden peas. Allele - an alternative form of a gene e.g. gene for height in pea plants has two forms -tall (T) or small (t). • Dominant gene: A gene that is expressed in the heterozygous condition • e.g. tall Tt. • Recessive gene: A gene that is expressed only in the homozygous condition • e.g. small tt. • Homozygous: The alleles of a gene pair are the same e.g. TT, tt - purebreeding. • • Heterozygous: The alleles of a gene pair are different (Tt) – ‘carriers’. Gregor Mendel • Father of genetics • 1857 – began collecting pure lines of peas • Chose self-fertilizing peas so that all offspring look exactly like their parent • Mendel chose 7 traits for study Pea Traits used by Mendel 14 Mendel’s Peas 1.Pure lines with easily identifiable traits were available 2.Peas are self-fertilizing with a flower structure that minimizes accidental pollination 3.Peas can be artificially fertilized which allows specific crosses to be made 4.Peas have a short growth period 5.Peas produce large numbers of offspring 15 (1) He crossed pure plants with alternative phenotypes for a single trait (2) He recorded how many offspring were of each type – 1st generation results (F1 generation) (3) He allowed these offspring to self-fertilize (4) He again recorded the nature of the offspring – F2 generation (5) He did a mathematical analysis (6) He deduced several principles (7) He published paper in good scientific journal – 1866 LAW OF SEGREGATION (MENDEL’S FIRST LAW): • A characteristic is controlled by 2 genes which separate at gamete formation. LAW OF INDEPENDENT ASSORTMENT (MENDEL’S SECOND LAW): • When gametes are formed, each member of a pair of alleles may combine randomly with either of another pair. Phenotype • The physical appearance of the organism • Examples: 1. tall pea plant 2. dwarf pea plant Genotype The set of genes an individual possesses Example: 1. tall pea plant TT = tall (homozygous dominant) 2. dwarf pea plant tt = dwarf (homozygous recessive) 3. tall pea plant Tt = tall (heterozygous) GENETIC CROSS TERMS • Progeny: offspring produced • Haploid: having a single set of unpaired chromosomes. • Diploid: having chromosomes in pairs. • Mitosis: Produces two identical daughter cells and maintains chromsome number during cell division of somatic cells. • Meiosis: Produces four daughter cells and reduces the chromsome number by half during cell division. • Gamete: a haploid sex cell. • A carrier of a trait has one copy of the recessive allele and therefore shows the dominant allele. Monohybrid Cross • a genetic cross in which only one characteristic is being examined e.g. TT x Tt Example of Results – Seed Coat (Smooth seeds vs Wrinkled seeds) Parents: • one parent had smooth seeds • the other wrinkled seeds Result: F1 all smooth 24 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 sperm B B Bb haploid (n) b diploid (2n) b b meiosis I meiosis II Monohybrid Cross • Example: Cross between two heterozygotes for brown eyes (Bb) BB = brown eyes Bb = brown eyes bb = blue eyes B b B Bb x Bb b female gametes male gametes Monohybrid Cross B b B BB Bb b Bb bb Bb x Bb 1/4 = BB - brown eyed 1/2 = Bb - brown eyed 1/4 = bb - blue eyed 1:2:1 genotype 3:1 phenotype Punnett square 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 t T tall (TT) vs. dwarf (tt) pea plants T T t Tt Tt produces the F1 generation t Tt Tt All Tt = tall (heterozygous tall) Breed the F1 generation • tall (Tt) vs. tall (Tt) pea plants T T t t tall (Tt) vs. tall (Tt) pea plants T T t TT Tt t Tt tt produces the F2 generation 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt 1:2:1 genotype 3:1 phenotype Dihybrid • A genetic cross where two contrasting traits are investigated • Eg: TtYy or TTYY • Example: Seeds may be yellow or green in colour. They may also be round or wrinkled in shape. Let; • • • • Y = yellow (dominant) y = green (recessive) R = Round (dominant) r - wrinkled (recessive) DIHYBRID CROSSES SHOW MENDELS 2ND LAW • Law of independent assortment (Mendel’s second 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) • YyRr YR • meiosis or Yr or yR or yr. • Inheritance of many human characteristics are in keep with Mendel’s findings e.g. eye colour, albinism, cystic fibrosis, blood groups. • 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. 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 R R Incomplete Dominance R R r Rr Rr produces the F1 generation r Rr Rr All Rr = pink (heterozygous pink) Pink Flowers? Co-dominance • Both alleles are dominant and both express their phenotype (multiple alleles) in heterozygous individuals. • Example: blood 1. 2. 3. 4. type A type B type AB type O = = = = IAIA or IAi IBIB or IBi IAIB ii Co-dominance • Example: homozygous male B (IBIB) x heterozygous female A (IAi) IB IB IA IA I B IA IB i IB i IB i 1/2 = IAIB 1/2 = IBi 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 • Linked genes are genes located on the same chromosome, which tend to be inherited together. It is important to note: • Independent assortment does not occur between linked genes 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 • • L • • Gametes:GL • • G • • L • F1 : • G g g L l l X gl g l GgLl Self-cross (if genes linked): • • Parents: GgLl • Gametes: GL • F2: • GGLL X gl GgLl GgLl GL gl GgLl ggll 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. • 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 X Gametes:X F1 genotype: Phenotype: X Father X Y (Egg) X orY (Sperm) XX Female XY 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. 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 • • X HX h • • Gametes: XH X h • Male normal X HY XH Y • • • • • • • F1 X HX H X HY X HX h Xh Y 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 2006 OL Q11 2008 HL Q11 • Haemophilia in humans is governed by a sexlinked allele. The allele for normal blood clotting (N) is dominant to the allele for haemophilia (n). • (i) What is meant by sex-linked? • (ii) Determine the possible genotypes and phenotypes of the progeny of the following cross: haemophilic male X heterozygous normal female. Non-nuclear inheritance Mitochondria and chloroplasts contain their own DNA, which indicates that they are descendants of once free-living bacteria. • At human fertilisation, only the head of the sperm enters the egg. Each offspring gets a nucleus from the male parent and a nucleus plus cytoplasm from the female parent. Mitochondria are inherited from the female only. Mitochondrial DNA has been used as a molecular clock to study evolution. By measuring the amount of mutation that has happened the time that has taken for it to occur can be calculated. • Mitochondria generate about 90% of the energy of the cell and other cells that use a lot of energy would be dependent on them. Other examples of non-nuclear inheritance include leaf variegation in snapdragons, Parkinson’s disease. Multiple alleles • More than two different forms of a gene e.g. blood grouping in man is governed by 3 alleles A, B and O. Only two of the possible number of alleles will be in any one organism. • Human Blood groups: • 3 allelic genes A, B and O. • A and B are co-dominant • O is recessive. Genotype AA AO BB BO AB OO Phenotype A A B B AB O End