Mendel and Genetic Crosses

Mendel Gregor Mendel – botanist Studied inheritance through pea plants 1850’s Pea plants – sexual reproduction Usually self-fertilize The same plant gives both male and female gametes Mendel had the plants self fertilize until they made the same traits over and over True Breeding: organisms with consistent traits generation after generation Then he selected plants to cross breed “cross pollination”

Mendel studied seven traits of pea plant P generation (parental)– “true breed” So he experimented with crossing one form of true breeding plant with another form of true breeding plant (ex one with green seeds and one with yellow seeds F1 generation- “first filial” (child plant) Called a monohybrid cross – 1 trait is examined (mono= one) and hybrid plants (made from parents with different traits

Oh- they were all yellow! Then, Mendel let the F1 generation (all yellow) self fertilize, and some green appeared in the next generation (F2 generation) So the green trait was not gone, but only “hidden”

Mendel realized- 2 hereditary “factors” for each trait Now we call them alleles (genes) Alleles are a different form of a gene So Mendel realized that the F1 generation (yellow seeds) had a copy of each allele in them – one for yellow, and one for green Since the F1 yellow had both alleles in them, he realized that yellow allele is dominant Green is recessive

Law of segregation Inherited traits are determined by pairs of ‘factors’– alleles of a gene The two alleles in a parent separate in the gametes (during meiosis) so each gamete would contain one or the other allele (yellow or green) When fertilized, the offspring would have one allele from each parent We can model this

Punnett Square

Let’s try a test cross Monohybrid cross

heterozygous- having two forms of the allele for a trait Homozygous- having 1 form of the allele for a trait Example: Yy– heterozygous YY- homozygous yy- homozygous

Inheritance of two traits Dihybrid crosses Then Mendel tested for two traits at once Yellow or green colour, and round or wrinkled seeds All F1 gen produced dominant forms of each trait– yellow and round traits Then when he let them self-fertilize, some of the recessive forms of each trait reappeared in the F2 generation

Law of Independent Assortment -dihybrid crosses show a ratio of 9:3:3:1 in the F2 generation That means that for every 1 offspring with both recessive traits (green and wrinkled (yyrr), there are 3 that have 1 recessive and 1 dominant (yellow and wrinkled Y-, rr) and 3 that show the other combo (green and round (yyR-) and 9 displaying dominant forms of both traits (yellor round Y-R-) **these ratios are true only if inheriting one trait has no influence on inheriting the other trait The law of independent assortment is when “two alleles for one gene segregate or assort independently of the alleles for the other gene” (this isn’t always true, but usually)

Genotype= the allele combination that an organism has Phenotype = the physical trait expressed Ex: Yy Genotype= one dominant allele yellow Y and one recessive allele green y Phenotype= yellow

Work through question 13, 14, 15 and 16 on p. 215 Read “Using a Punnett Square to Analyze a Dihybrid cross Practice problems p , 12, 13, 14, 15, 16, 17, 19, 20