DIHYBRID CROSSES & GENE LINKAGE AHL Topic 10.2 IB Biology Miss Werba

DIHYBRID CROSSES & GENE LINKAGE POLYGENIC INHERITANCE AHL TOPIC 10 – GENETICS 10.1 MEIOSIS 10.2 DIHYBRID CROSSES & GENE LINKAGE 10.3 POLYGENIC INHERITANCE J WERBA – IB BIOLOGY 2

THINGS TO COVER Genotype & phenotype ratios Autosomes v Sex chromosomes Crossing over = exchange of alleles Linkage group Dihybrid cross between two linked genes Identification of recombinants J WERBA – IB BIOLOGY 3

DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT 10.2.1 DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT Linked genes: Genes that occur on the same chromosome Likely to be inherited together Unlinked Genes: Genes that occur on separate chromosomes Follow Mendel’s Law of Independent Assortment - inheritance of alleles is random J WERBA – IB BIOLOGY 4

DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT 10.2.1 DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT The basic principles applied to solving monohybrid problems is also used for dihybrid problems. The only difference is that 2 traits are considered at the same time, rather than one. J WERBA – IB BIOLOGY 5

DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT 10.2.1 DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT Steps: Assign letter symbols for alleles Determine the parental genotypes Work out the possible allele combinations for each parent Draw up the Punnett square & complete. Determine genotype ratio – by counting the different genotypes Determine the phenotype ratio – by determining the possible phenotypes from the genotypes J WERBA – IB BIOLOGY 6

DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT 10.2.1 DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT Example: Two traits will be considered in Drosphilia flies: Eye colour Wing length These traits are coded for by unlinked genes. Assume that: Red eyes is dominant over pink Long wings are dominant over short J WERBA – IB BIOLOGY 7

DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT 10.2.1 DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT Q: A fly with pink eyes and short wings was crossed with a pure breeding one that had red eyes and long wings. Determine the genotype and phenotype ratios in the F1 and F2 generations. J WERBA – IB BIOLOGY 8

DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT 10.2.1 DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT Example: Let: R = red eyes, r = pink eyes L = long wing, l = short wings Parents are: pink eyes and short wings : possible gametes: red eyes and long wings: rrll rl RRLL RL J WERBA – IB BIOLOGY 9

DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT 10.2.1 DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT Example: F1 Genotype ratio: F1 Phenotype ratio: rl RL RrLl 100% RrLl 100% red eyes, long wings J WERBA – IB BIOLOGY 10

DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT 10.2.1 DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT Example: If F1 generation are left to interbreed: Possible gametes formed are: RL Rl rL rl RrLl x RrLl J WERBA – IB BIOLOGY 11

DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT 10.2.1 DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT Example: F2 Genotype ratio: RRLL; RRll; rrLL; rrll RrLl; rrLl; Rrll; RrLL; RRLl F2 Phenotype ratio: 9 red eyes, long wings 3 red eyes, short wings 3 pink eyes, long wings 1 pink eyes, short wings RL Rl rL rl RRLL RRLl RrLL RrLl RRll Rrll rrLL rrLl rrll J WERBA – IB BIOLOGY 12

DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT 10.2.1 DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT Example: In this cross the parents were: red eyes, long wings and pink eyes, short wings. The recombinants are: red eyes, short wings pink eyes, long wings The recombinant ratios observed are: 9 red eyes, long wings (same as parent genotype) 3 red eyes, short wings (recombinant) 3 pink eyes, long wings (recombinant) 1 pink eyes, short wings (same as parent genotype) J WERBA – IB BIOLOGY 13

DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT 10.2.1 DIHYBRID CROSSES (UNLINKED GENES) Command term = CALCULATE and PREDICT When two pure breeding individuals are crossed, the F1 will all have the same genotype and phenotype. They will all be heterozygous. If allowed to interbreed, the F2 will have a ratio of: 9 : 3 : 3 : 1 This is the same ratio that Gregor Mendel found over 300 years ago in his studies of pea plants. J WERBA – IB BIOLOGY 14

CHROMOSOMES Command term = DISTINGUISH 10.2.2 CHROMOSOMES Command term = DISTINGUISH Chromosome: molecule of DNA found in cell nuclei Homologous chromosomes: two chromosomes which have the same sequence of genes Autosome: a chromosome which determines the characteristics of an organism, except for its sex Sex chromosomes: determine gender in humans (XX= ♀; XY=♂) J WERBA – IB BIOLOGY 16

CHROMOSOMES Command term = DISTINGUISH 10.2.2 CHROMOSOMES Command term = DISTINGUISH Human body cells have 46 chromosomes. Exist as 23 homologous pairs (½ from each parent) One pair are the sex chromosomes (allosomes): Determine gender and some non-sexual traits Females XX Males XY 22 pairs are autosomes (determine non-sexual traits) You get 22 autosomes and 1 sex chromosome from each parent J WERBA – IB BIOLOGY 17

CROSSING OVER Command term = EXPLAIN 10.2.3 CROSSING OVER Command term = EXPLAIN Crossing over and recombination: when chromosomes exchange segments during meiosis, giving rise to variation Crossing over and recombination occurs during...? Prophase I of meiosis Crossing over and recombination occurs at the...? chiasma J WERBA – IB BIOLOGY 18

CROSSING OVER Command term = EXPLAIN 10.2.3 CROSSING OVER Command term = EXPLAIN When homologous chromosomes line up during prophase I they form a...? synapsis The pair of homologous chromosomes is referred to as a...? bivalent Portions of the adjacent chromosomes may swap, resulting in the formation of ...? recombinants or recombinant chromosomes J WERBA – IB BIOLOGY 19

CROSSING OVER Command term = EXPLAIN 10.2.3 CROSSING OVER Command term = EXPLAIN J WERBA – IB BIOLOGY Ref: Biology Key Ideas 20

LINKAGE GROUP Command term = DEFINE 10.2.4 LINKAGE GROUP Command term = DEFINE Linked genes are pairs or groups of genes which are carried on the same chromosome and are inherited together. Mendel’s law of independent assortment is not always true for genes on the same chromosome. It is still possible for recombination to occur between linkage groups, but it is far less likely. Gene linkage can occur on the autosomes and sex chromosomes. J WERBA – IB BIOLOGY 21

GENE LINKAGE Command term = EXPLAIN 10.2.5 GENE LINKAGE Command term = EXPLAIN The distance between two gene loci is measured as a recombination fraction (θ). It is measured in CentiMorgans (cM) – equivalent to a 1% chance of recombination. If two loci are not linked, then θ = 0.5 indicating that the genes separate together in 50% of all meioses If two loci are linked, then θ = 0.1 indicating that the genes separate together in 90% of all meioses and crossing over rarely only (only 10%) J WERBA – IB BIOLOGY 22

GENE LINKAGE Command term = EXPLAIN 10.2.5 GENE LINKAGE Command term = EXPLAIN Unlinked genes: are found on different chromosomes homologous pairs line up independently of each other during metaphase I of meiosis Independent Assortment allows for recombination T t B b T and B not linked J WERBA – IB BIOLOGY 23

GENE LINKAGE Command term = EXPLAIN 10.2.5 GENE LINKAGE Command term = EXPLAIN Linked genes: are found on the same chromosome can only be broken apart by crossing over and gene recombination during prophase I of meiosis T t B b T and B are linked J WERBA – IB BIOLOGY 24

GENE LINKAGE Command term = EXPLAIN 10.2.5 GENE LINKAGE Command term = EXPLAIN T t B b T and B not linked T t B b T and B are linked Alleles are normally shown side-by-side in dihybrid crosses TtBb Alleles in linkages groups are shown as vertical pairs: This means that T is linked to B (on same chromosome) and t is linked to b (on same chromosome). J WERBA – IB BIOLOGY 25

GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY 10.2.5 10.2.6 GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY A test cross was conducted on a fly heterozygous for eye colour and body colour . The results are shown below: 151 wild-type (normal colour eyes & normal body colour) 8 purple eyes and normal body colour 10 normal eyes and black body colour 131 purple eyes and black body colour What is a test cross? Explain how these F1 offspring support the idea that genes P and B are linked. J WERBA – IB BIOLOGY 26

GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY 10.2.5 10.2.6 GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY What is a test cross? a cross with a homozygous recessive genotype to allow the genotype of unknowns to be inferred Explain how these F1 offspring support the idea that genes P and B are linked. the genes did not move independently of one another; only crossing over & recombination will allow you to get the other 2 phenotypes in smaller numbers J WERBA – IB BIOLOGY 27

GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY 10.2.5 10.2.6 GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY Draw a chromosome picture for the heterozygous parent. Which F1 phenotypes represent: parental-type offspring? recombinant offspring? Calculate the percentage crossing over or recombination fraction (θ). If a recombination fraction of 1% is equivalent to one map unit (used to map chromosomes), calculate the map distance between genes P and B. J WERBA – IB BIOLOGY 28

GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY 10.2.5 10.2.6 GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY Draw a chromosome picture for the heterozygous parent. Which F1 phenotypes represent: parental-type offspring? 151 + 131 (normal eyes & normal body and purple eyes & black body) recombinant offspring? 8 + 10 (purple eyes & normal body and normal eyes & black body) P B p b J WERBA – IB BIOLOGY 29

GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY 10.2.5 10.2.6 GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY Calculate the percentage crossing over or recombination fraction (θ). If a recombination fraction of 1% is equivalent to one map unit (used to map chromosomes), calculate the map distance between genes P and B. 6%  6 map units apart J WERBA – IB BIOLOGY 30

GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY 10.2.5 10.2.6 GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY Predict the genotypes of the offspring (and their expected frequency) for the following mating: J WERBA – IB BIOLOGY 31

GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY 10.2.5 10.2.6 GENE LINKAGE EXAMPLES Command term = EXPLAIN and IDENTIFY 20 map units apart  20% recombination fraction (θ= 0.2) Parents: x Gametes: AB, ab x ab F1 genotype ratio: parental-type offspring – ; recombinant offspring – ;  must add up to 80% (ie. 40% each)  add up to 20% (ie. 10% each) J WERBA – IB BIOLOGY 32

Sample questions Q1. J WERBA – IB BIOLOGY 33

Sample questions Q2. In some maize plants the seed is enclosed in a green sheath called a tunica. The allele (T) for this is dominant to the allele (t) for normal, unenclosed seeds. The endosperm of the seed can be starchy (allele E) or sugary (allele e). The genes for these two characteristics are linked. The table below shows the outcome of crosses between a plant heterozygous for both characteristics and one that is homozygous recessive for both characteristics. J WERBA – IB BIOLOGY 34

Sample questions Q2. State the genotype of the heterozygous parent using the correct notation. [1] Identify which individuals are recombinants in this cross. [1] Explain what has occurred to cause these results. [2] Maize belongs to the group of plants known as angiospermophyta. Distinguish between angiospermophytes and bryophytes. [2] J WERBA – IB BIOLOGY 35

Sample questions A1. C A2. unenclosed seeds, starchy and tunica present, sugary and (both needed) J WERBA – IB BIOLOGY 36

Sample questions A2. crossing over; between non-sister chromatids (in prophase I); results in exchange of alleles / change in linkage groups; so some gametes are T e or t E; test cross expect ratio of two phenotypes / correct Punnett Square showing test cross; but instead get four phenotypes with smaller percentage of recombinants; Above points can be shown in diagrams. J WERBA – IB BIOLOGY 37