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 #1Allele #2Phenotype Dominant Dominant Trait Shows DominantRecessiveDominant Trait Shows RecessiveDominantDominant Trait Shows 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. TTtt

Our Parental Pea Plants FatherMother TTtt End of Meiosis Splits the two alleles So we can have ½ chromosome count in our sex cells (gametes) TT tt spermeggs

What are the possible zygotes of the F1 Generation produced by crossing parents? TTtt sperm eggs Tt All heterozygous tall zygotes

What if we crossed 2 heterozygous tall plants? Tt Tt Tt TTTt tt Possible Gametes X

So in our F2 Generation we get… TTTt tt Three tall pea plants and…. One short! 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.

TTTt tt Genotype Ratio:TT1 Tt2 tt1 1 TT : 2 Tt : 1 tt Phenotype Ratio: 3 tall 1 short 3 tall : 1 short