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| Understanding Genetics |
| Step 1. Go here to review the commonly used terms and definitions |
| Since eggs and sperm each carry only one of each chromosome instead of a pair of each, they carry only one copy of each gene instead of two. Thus a female who carries two different flavors (alleles) for a particular gene, produces some eggs reproductive cells that carry one flavor, and some eggs that carry the other. A male with two different alleles for the same gene likewise produces some sperm carrying one allele and some sperm carrying the other. This is the basis of the Punnett square - line up the possible reproductive cells for one parent along the top of the square, and the possible reproductive cells for the other along the left side of the square, then combine them in the middle to show all possible combinations of alleles of this genes from the two parents that may occur in their offspring. Creating the Punnett Square You begin by drawing a grid of lines as below: Next, put the genotype of one parent across the top and that of the other parent down the left side. For example, if parent genotypes were TT ( Regular) which is a dominant Trait and tt (recessive trait of Tangerine), the setup would be as below. Note that only one letter goes in each box for the parents. It does not matter which parent is on the side or top of the square. Next, all you have to do is fill in the boxes by copying the row and column-head letters across or down into the empty squares. This gives us the predicted genotypes among the offspring each time reproduction occurs. In this example, 100% of the offspring will likely be Regular in appearance and Heterozygous (Tt) for Tangerine because only one parent was carrying that gene and it takes two genes to show up physically. In the next example (shown below), if the parent both have heterozygous (Tt) genotypes, there will be 25% TT, 50%Tt, and 25% tt offspring on average. These percentages are determined based on the fact that each of the 4 offspring boxes in a Punnett square is 25% (1 out of 4). As to phenotypes, 75% will be Regular in appearance and only 25% will be Tangerine. In summary , An offspring's genotype is the result of the combination of genes that came together in its conception. One sex cell came from each parent. Cells normally only have one copy of the gene for each trait (ex. one copy of the T or t form of the gene in the example above). Each of the two Punnett square boxes in which the parent genes for a trait are placed (across the top or on the left side) represents one of the two possible genotypes for a parent sex cell. Which of the two parental copies of a gene is inherited depends on which sex cell is inherited. It is a matter of chance. By placing each of the two copies in its own box has the effect of giving it a 50% chance of being inherited. Breeding Expectations at a Glance: Can you do the Punnett Squares? PAIR #1 - Homozygous X Homozygous (albino x albino) aa X aa Every baby is going to get a recessive (albino) gene from each parent, because that's all each parent has to contribute, so all the babies will end up with TWO albino genes, making all of them albinos PAIR # 2 - Homozygous X Heterozygous (Axanthic x het/Axanthic) cc X Cc =1/2 homozygous =1/2 heterozygous Every baby will get a recessive (Axanthic) gene from the homozygous parent, because that's all that parent has to contribute; The het parent will contribute Axanthic genes half the time, and normal genes half the time, So half the babies will get two Axanthic genes and will be Axanthic ( lacking yellow pigment), and the other half will get one Axanthic gene and one normal gene and will be hets, which look normal but carry the trait which can be passed on to offspring) PAIR #3 - Heterozygous x Heterozygous (het/albino x het/albino) Aa x Aa =1/4 homozygous (Albino) =1/2 heterozygous (het/Albino) =1/4 normal The father, who has one Albino gene and one "normal" gene, will give albino genes to half the babies and normal genes to the other half; the mother will do likewise. Half the babies that got albino genes from the father will get an albino gene from the mother, too, so they will have two Albino genes and will be Albino. They'll make up 1/4 of the offspring (half of half). Half of the babies that got normal genes from the father will get normal genes from the mother, too, so will have two normal genes, and will be normal. They'll make up 1/4 of the offspring. The other half-of the babies get an albino gene from one parent and a normal gene from the other, so are hets. Hets and the normals look alike and, combined, make up 3/4 of the production. But two out of three of those normal-looking babies are actually hets, so we call those normal-looking babies possible hets with 2/3 chance of being het, or "2/3 chance possible hets". They are either hets or they're not, but we don't know which are which, so we use terminology that correctly characterizes the likelihood of their being hets. |
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