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Understanding How Gametes are Formed Alleles • First remember all of our traits or genes come in pairs. • Remember if you have a pair of jeans – you have two pant legs correct? • Same goes for our genes (genetic traits). • ½ of the gene called an allele, comes from our father. • The other ½ of the gene also called an allele, comes from out mother. • Two alleles together create our homologous chromosome or “twins”. • What this means is if I have a chromosome that codes for hair color from my father’s sperm, when it unites with my mother’s egg that particular chromosome will look for its “twin” – hair color chromosome from mother. • A chromosome coding for height lets say won’t go pairing up with one coding for eye color….etc. • In addition we all have either dominant or recessive alleles. • How those alleles pair together determines what we actually “show” – phenotype. • If your dad gives you a dominant trait and your mother gives you the recessive trait – you will show the dominant trait… Allele #1 Allele #2 Phenotype Dominant Dominant Dominant Trait Shows Dominant Recessive Dominant Trait Shows Recessive Dominant Dominant Trait Shows Recessive Recessive Recessive Trait Shows • Ok, so if we crossed two pure plants, one pure for the tall trait and one pure for the short trait… • Tall is dominant for height – T • Short is recessive for height – t • To easily see how the genotypes and phenotypes work together, if the phenotype shows the dominant trait it will be colored blue, and if it shows the recessive trait it will be colored pink. TT tt Our Parental Pea Plants End of Father Meiosis Splits the two alleles TT So we can have ½ chromosome T T count in our sex cells sperm (gametes) Mother tt t t eggs What are the possible zygotes of the F1 Generation produced by crossing parents? eggs sperm T T Tt Tt t Tt All heterozygous tall zygotes t Tt What if we crossed 2 heterozygous tall plants? X Tt Tt Possible Gametes T t TT Tt T t Tt tt So in our F2 Generation we get… Three tall pea plants and…. TT Tt Tt One short! tt How is this if heterozygous shows the dominant trait? Because heterozygous not only carries a dominant trait – IT ALSO CARRIES THE RECESSIVE! The right combo can result in a homozygous recessive offspring showing recessive trait. TT Tt Genotype Ratio: 1 TT : 2 Tt : 1 tt Phenotype Ratio: 3 tall 1 short 3 tall : 1 short Tt TT Tt tt 1 2 1 tt
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