Dihybrid crosses and gene linkage

Dihybrid crosses and gene linkageTopic 10.2Assessment Statements10.2.1 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.10.2.2 Distinguish between autosomes and sex chromosomes.10.2.3 Explain how crossing over between non-sister chromatids of a homologous pair in prophaseI can result in an exchange of alleles.10.2.4 Define linkage group.10.2.5 Explain an example of a cross between two linked genes.10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.Dihybrid crossesObservations: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)Other parent: homozygous recessive for both traits (wrinkled and green)R = allele for round peasr = allele for wrinkled peasY = allele for yellow peasy = allele for green peasParent phenotypesRound yellowGreen wrinkledParent genotypesRRYYrryyParent gametesRYryF1 genotypesRrYyF1 phenotypesRound yellowAllowing heterozygous offspring to self-pollinate yields following proportions:Round and yellow 56.6%Wrinkled and yellow 18.2%Round and green 19.4%Wrinkled and green 8%Ratio9:3:3:1

Mouse GeneticsAutosomes and sex chromosomesSex chromosomes: X and Y (one pair)Autosomes: any chromosome not X or Y (22 pairs)Sex-linked gene or trait is located on a sex chromosome.Autosomal gene or trait is located on one of the autosomes.On which type of chromosome is the gene for protein production in the testes found?So, the gene is known as ___.On which type of chromosome is the gene for protein production in the pancreas found?So, the gene is known as ___.

Exchange of alleles by crossing overTwo non-sister chromatids can swap segments of their DNAA maternal chromosome can end up with a segment of a paternal chromosome and vice versa

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 Mendels law of independent assortmentLinked genesFruit fly gene for body color is in the same linkage group as the gene for wing lengthG grey bodyg black bodyL long wingsl short wingsGGLL gray body, long wingsggll black body, short wings

G LG LTwo horizontal bars symbolize homologous chromosomes and that the locus of G is on the same chromosome as LF2 generation genes located on same pair of chromosomes, so only possible gametes are PL and plPhenotypic ratio is 3:1

Offspring of a dihybrid crossA cross between homozygous dominant true-breeding fruit fly (GGLL) and a homozygous recessive true-breeding fruit fly (ggll) would result in flies which were all heterozygous for both of the traits (GgLl)If heterozygotes for these traits are mixed with homozygous dominant flies, then one could not determine the genotype by sight.What to do?

Test CrossCross mystery fly with homozygous recessive (ggll)Assuming the mystery fly is heterozygous (GgLl) for both traits:

GLGlgLglglGgLlGgllggLlggllA new shuffling of the alleles has created a new combination which does not match either of the parents genotypesThe term recombinant is used to describe both the new chromosome and the resulting organism.Recombinants form through the process of crossing over

Polygenic InheritanceTopic 10.3Assessment Statements10.3.1 Define polygenic inheritance.10.3.2 Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour.

DefinitionInvolves two or more genes influencing the expression of one traitIncreased number of possible genotypesBelieved that most human traits are too complex to be determined by one gene

Continuous and discontinuous variationWhen 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, widows peak, blood typeGraphical representationContinuousvariationDiscontinuous variation

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.

Skin colorMany societies feel the need to label people with categoriesOversimplification serves more administrative purposes than any biological purposeSo what is the purpose of skin?!PROTECTION FROM THE SUNS HARMFUL ULTRAVIOLET RADIATION!!!Purpose of melanin (skin pigment)Melanin protects from serious sunburn which can lead to melanomaMelanin is found in all individuals except albinosConcentration of melanin variesHow can one increase the melanin level in their skin?EXPOSE ONESELF TO SUNLIGHTTanning is a natural defense against the negative effects of excess sunlightUV light and calciferol (vitamin D)UVB radiation helps the skin to make vitamin DVitamin D is necessary for proper growth and bone formationIt is good to have a MODERATE amount of sunlightIn regions of low Sun exposure people need light-colored skinWhy?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?Chart11015501510

FrequencyHeight in humans

Sheet1HeightFrequency4.5105155.5506156.510

Chart14012840

% of populationBlood Type

Sheet1% of populationSeries 2Series 3A402.42B124.42AB81.83O402.85To resize chart data range, drag lower right corner of range.