Chapter 17 Genetics Crosses:

Transcription

1 Chapter 17 Genetics Crosses: 2.5 Genetics Objectives Genetic Inheritance H Origin of the Science of genetics H Law of segregation H Law of independent assortment H Dihybrid cross 1. Define a gamete and discuss their formation 2. Define fertilisation 3. Discuss sex determination 4. Define allele and differentiate between the terms homozygous and heterozygous 5. Differentiate between genotype and phenotype 6. Differentiate between dominant and recessive 7. Explain incomplete dominance 8. Be able to complete monohybrid crosses and state the genotypes and phenotypes of parents and offspring 9. Understand the 3:1 ratio for heterozygous crosses 10. Discuss the work of Gregor Mendel 11. State the Law of Segregation 12. Discuss the chromosomal basis for this law 13. Complete monohybrid crosses 14. State Mendel s Law of Independent Assortment 15. Discuss the chromosomal basis for this law 16. Complete dihybrid crosses 17. Know the expected ratios from Mendel s Laws 18. Define linkage and identify linked genes 19. Define sex linkage and give examples of sex linked characteristics 20. Discuss non-nuclear inheritance Somatic cells consist of all of the body cells except the sex cells, they are diploid (2n) and contain 46 chromosomes. Gametes are sex cells. They are haploid cells (n) which are capable of fusion. In humans gametes are the egg and sperm. Their function is to pass genes from one generation to another during sexual reproduction. - Gametes are made during meiosis (chromosome number is halved allowing fertilisation to occur). Fertilisation is the union of two gametes to form a single cell called a zygote Your genes work with your environment to determine your traits/characteristics. Genotype is the genetic make-up of an organism. Phenotype is the physical make-up or appearance of an organism. Phenotype = Genotype + Environment

2 Diploid cells contain 2 copies of each chromosome, this means 2 copies of every gene. Alleles are different forms of the same gene. - The locus of a gene is its position on a chromosome. If a cell has two of the same alleles they are said to be homozygous, if they have two different alleles they are heterozygous. Homozygous genes have two identical alleles for the same trait. Heterozygous genes have two different alleles for the same trait. A Dominant allele determines the phenotype. Dominant alleles prevent recessive alleles from being expressed. If an allele is recessive it is prevented from being expressed by the dominant form. Possible genotypes include: - Heterozygous/Incompletely Dominant (Bb) - Homozygous Dominant (BB) - Homozygous Recessive (bb) Codominance/Incomplete dominance means neither allele is dominant or recessive with respect to the other. Both alleles are equally expressed in the heterozygous genotype to produce an intermediate phenotype. Genetics crosses involve following the genes of both parents and how they combine to produce offspring. - A Punnett Square is used to show the ratio of the offspring of a genetic cross. - Progeny is the offspring of a cross. - F1 is the first filial generation, i.e. the first generation of offspring - F2 is the second filial generation, i.e. if you cross an F1 with another

3 Monohybrid Crosses- One characteristic/trait Homozygous Cross In pea plants, green pods (G) is dominant over yellow pods (g). Give the genotypes and phenotypes for the offspring of a cross involving two homozygous pea plants whose genotypes are (GG) and (gg). Heterozygous Cross In Humans, the gene for brown eyes (B) is dominant to that for blue eyes (b). Give the genotypes and phenotypes for the offspring of a cross involving two heterozygous parents. Incomplete Dominance Cross

4 A Pedigree is a diagram showing the genetic history of a group of related individuals. Every somatic cell in the body contains 46 chromosomes are autosomes - 2 are sex chromosomes - Autosomes control all non-gender related features and traits. - The sex chromosomes are X and Y. They control gender related traits. - In humans females are XX and males are XY. Y is half the size of X. - It is the male who determines the sex of the child. - The ratio of male to female births should be equal, 50/50, 1:1 The cross to show this ratio is: Gregor Mendel Gregor Mendel is known as the father of genetics. Mendel carried out experiments on pea plants and studied the inheritance of 7 characteristics in them, including flower colour, stem height, and seed shape. - His experiments involved transferring pollen between flowers by hand, and growing the seeds in labelled pots to see which trait (phenotype) was passed on. - His success was down to two key features.

5 He only studied features that displayed two forms: yellow or green, tall or small, round or wrinkled. He counted the number of plants with each type of trait and calculated mathematical ratios around how traits were inherited: 1:1 or 3:1 etc. Mendel s Law s 1 st Law of Segregation Inherited characteristics are controlled by pairs of alleles. These alleles segregate from each other at gamete formation, with only one member of the pair being found in each gamete. Chromosomal Basis of 1 st Law: In diploid cells chromosomes occur in homologous pairs. Alleles are located at the same locus on each chromosome in the pair. During meiosis, homologous chromosomes separate into different haploid cells. This means the alleles separate. 2nd Law of Independent Assortment When gametes are formed, either pair of alleles, is equally likely to combine with either of another pair of alleles. Chromosomal Basis of 2 nd Law: During meiosis (gamete formation) each homologous chromosome is equally likely to combine with any other homologous chromosome from a pair. Dihybrid Crosses- Two characteristics/traits Both parents homozygous for both traits In Pea-plants, tall plant (T) is dominant over small plant (t). In addition green pod (G) is dominant over yellow pod (g). A tall plant with green pods (homozygous for both traits) is crossed with a small plant with yellow pods. Show the genotypes and phenotypes of the F1 progeny of this cross.

6 One parent heterozygous, one parent homozygous recessive for both traits In Guinea-pigs black coat (B) is dominant to white coat (b). Also short hair (S) is dominant to long hair (s). Show the expected genotypes and phenotypes of the progeny from a cross involving a black-coated, short-haired guinea-pig (heterozygous for both traits) and a white-coated, long-haired animal. A selfed (F2) cross A homozygous purple-flowered, short-stemmed plant is crossed with a red-flowered, long-stemmed plant. All resulting progeny (F1) were purple-flowered, short-stemmed plants. (a) State the dominant and recessive traits, (b) Explain using diagrams, why all the F1 progeny have the same phenotype, (c) Give the expected phenotype ratios if an F1 plant is selfed.

7 Linkage Linkage means that genes are located on the same chromosomes. Linked genes as a result do not undergo independent assortment, they tend to be passed onto the next generation together. Non-linked genes Linked genes Show the expected genotypes of the progeny for the following cross, AaBb X aabb. (a) where there is no linkage, (b) where genes are linked A to B and a to b. Sex Linkage means that a characteristic is controlled by a gene on a sex (or X) chromosome. The sex chromosomes are the X and Y chromosomes. The X chromosome is much larger than the Y and thus carries many more genes. Sex linked traits are also called X-linked as they are carried on the X chromosome. There is no corresponding allele on the Y chromosome. The recessive phenotype is more common in males, that is they are more likely to have X-linked traits/diseases. Sex linked characteristics include colour-blindness, Haemophilia and Muscular Dystrophy. Colour-blindness Normal individuals can detect three colours of light (red, green and blue).

8 Normal is the dominant trait (N), colour-blindness is the inability to distinguish between red and green and is the recessive trait (n). Females can have three genotypes for colour-blindness. - For a female to be colour blind she must inherit the recessive allele from both parents, that means the mother must be a carrier and the father colour-blind XNXN XNXn XnXn Because the allele for colour vision is carried on the X chromosome males only have one allele. This means there are only two genotypes possible in males. - Males only need one recessive allele to be colour-blind so it is much more likely than in females. Haemophilia XNY- Haemophilia is a disorder where people cannot properly clot blood and can bruise very easily and may bleed to death from a very small cut. XnY- As with any other X-linked trait it is very rare in females as they have to inherit two recessive alleles (carrier mother, haemophiliac father). Males only have one allele on the X chromosome, thus only need to inherit one recessive allele (carrier mother). N is the dominant normal trait while n is the recessive disease-carrying trait. Non-nuclear inheritance Most DNA is contained in the nucleus of the cell, however mitochondria and chloroplasts (in plants) each contain one single strand of DNA. The DNA in mitochondria is present in female eggs but is only present in the tail of sperm, hence mitochondrial DNA is only passed on from the mother (maternal inheritance).