Presentation on theme: "Dihybrid crosses and gene linkage"— Presentation transcript:
1 Dihybrid crosses and gene linkage HL Genetics Topic 10.2
2 Assessment Statements Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.Distinguish between autosomes and sex chromosomes.Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.Define linkage group.Explain an example of a cross between two linked genes.Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.
3 Dihybrid crosses Mendel’s Peas Seed shape – some round, others wrinkled (allele for round is dominant)Seed colour – some green, others yellow (allele for yellow is dominant)Mendel crossed true breeding plants with each otherOne parent: homozygous dominant for both traits (round and yellow seeds) RRYYOther parent: homozygous recessive for both traits (wrinkled and green) rryy
4 When both parents are homozgous – all the F1 offspring are the same genotype and phenotype Parent phenotypesRound yellowGreen wrinkledParent genotypesRRYYrryyParent gametesRYryF1 genotypesRrYyF1 phenotypesR = allele for round peasr = allele for wrinkled peasY = allele for yellow peasy = allele for green peas
5 Cross the F1 double heterozygous Allowing heterozygous offspring to self-pollinatePhenotype Ratio is:931
6 Dihybrid Phenotype Ratios Homozygous parents AABB x aabb All F1 offspring the sameHeterozygous parents AaBb x AaBb 9:3:3:1 phenotype ratio 9AB 3Ab 3aB 1abA new shuffling of the alleles has created a new combination which does not match either of the parents’ genotypes. Recombinants
7 Autosomes and sex chromosomes Sex chromosomes: X and Y (one pair)Autosomes: any chromosome not X or Y (22 pairs)Sex-linked gene is located on a sex chromosome.Autosomal gene is located on one of the autosomes.On which type of chromosome is the gene for haemophilia found?So, the gene is known as ___.On which type of chromosome is the gene for protein production in the pancreas found?
8 Exchange of alleles by crossing over Two non-sister chromatids can swap segments of their DNA during prophase I of meiosis.This increases genetic variety of chromosomes in gametes
9 There are now thought to be 20,500 human genes on 23 chromosomes Source: Broad Institute of MIT and Harvard (2008, January 15). Human Gene Count TumblesThere must be approx 1000 genes on each chromosome.
10 Linkage groupAny two genes which are found on the same chromosome are said to be linked to each other.Linked genes are usually passed on to the next generation together.Linkage group - groups of genes on the same chromosome inherited togetherLinked genes are the exception to Mendel’s law of independent assortment
11 Linked genesFruit fly gene for body color is in the same linkage group as the gene for wing length Alleles are G – grey body g – black body L – long wings l – short wingsNotation of linked genes is G L The two horizontal bars symbolize homologous chromosomes and that the locus of G is on the same chromosome as LG LG L
12 Offspring of a dihybrid cross A cross between homozygous fruit fliesGGLL x ggllGrey body & long wings x Black body, short wingsF1 flies will be all heterozygous for both of the traitsGgLlPhenotype : Grey body & long wingsIf these F1 heterozygotes were allowed to reproduce together the ratio of phenotypes produced would be expected to be9:3:3:1
13 Recombinants After the F2 flies were identified the phenotypes were; Grey long wingGrey short wingBlack long wingBlack short wing16001020210200170This is far from the expected 9:3:3:1 ratiowhich should giveThis is a sign that the genes are on the same chromosome (genes are linked)
14 Offspring of a dihybrid cross using linkage notation A cross between homozygous fruit fliesGL glGL x glGrey body & long wings x Black body, short wingsF1 flies will be all heterozygous for both of the traitsGLglPhenotype : Grey body & long wingsIf these F1 heterozygotes were allowed to reproduce together the ratio of phenotypes produced would be expected to be9:3:3:1
15 Offspring of a dihybrid cross using linkage notation If these F1 heterozygotes make gametesThe gametes will beGL or glunless crossing over happens – because the genes are on the same chromosomeThe F1 will contain more than expected of these genotypesGL GL gl glGL gl GL gl
16 Offspring of a dihybrid cross using linkage notation If The F1 will contain more than expected of these genotypes The Grey short wing and black long wing phenotypes Are Recombinants -Grey long wingGrey short wingBlack long wingBlack short wing16001020210200170GL GL gl glGL gl GL gl
18 Assessment Statements Define polygenic inheritance.Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour.
19 Polygenic Inheritance when two or more genes influence the expression of one traitEg Skin Colour, Height
20 Continuous and discontinuous variation When an array of possible phenotypes can be produced, it is called continuous variationExamples: skin color, height, body shape, and intelligenceThese traits are also influenced by environmental conditionsWhen only a number of phenotypes can be produced, it is called discontinuous variationExamples: earlobe attachment, blood group
22 Eye ColorIris is made up of zones, rings, streaks or speckles of different colored pigments with varying intensitiesWhat color are your eyes, really?Since there is so much variety, eye color must be influenced by multiple alleles and has continuous variation.
23 Skin colorHow does the existence of multiple alleles controlling skin colour result in the appearance of many different shades of skin colour in humans?
24 ThoughtsHow do people of varying degrees of skin color relocated to parts of the world that receive differing amounts of sunlight get vitamin D? How do others fight off the sun?Should there be equal esteem for all humans?Why is human diversity so often used to divide and discriminate, rather than be appreciated, respected, and celebrated?
25 Polygenic inheritance of color in wheat. Kernal color in wheat is determined by two genes. A range of colors occur, from white to dark red, depending on the combinations of alleles. Dark red plants are homozygous AABB and white plants are homozygous aabb.
26 the F1 offspring are all double heterozygous AaBb. Crossing individuals with the phenotype extremes yield offspring that are a 'blend' of the two parents.When these homozygous phenotypes are crossedAABB x aabbDark x whitethe F1 offspring are all double heterozygous AaBb.
27 But what happens when the two double heterozygous genotypes are crossed? Parent Phenotypes: all brownGenotypes: AaBb x AbBbPunnet square:There is no blending in the offspring. Offspring can be more extreme than either parentThere are grades of colour – evidence of polygenic inheritance.ABAbaBabAABBAABbAaBBAaBbAAbbAabbaaBBaaBbaabb
28 Skin colorHow does the existence of multiple alleles controlling skin colour result in the appearance of many different shades of skin colour in humans?
29 Human skin colour is controlled by multiple alleles (and the environment) It is known that at least three genes control skin color, let’s call them genes A, B, and C.Someone who is AABBCC would have very dark skin color and someone who is aabbcc would have very light skin color.If they got married and had children, their children would all be AaBbCc and have mid-brown skin.If two of those people would get married and have children, the Punnett square would look like the one above.