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Transcript of Mendelian Genetics
What you need to know Mendel's Experiment Y y
Y YY Yy
y Yy yy Green color did not disappear
It was hidden in the F1 generation
Green reappeared in the F2 generation
Green is recessive to yellow If you have a cross between a homozygous yellow pea and a homozygous green pea, why would all of the F1 generation have yellow phenotypes? Allele: One of several possible forms of a particular gene Genotype: A set of alleles that determines the expression of a particular trait Phenotype: Actual physical appearance of an individual or organism Homozygous: Having identical alleles for a given trait; YY, yy, AA, aa Heterozygous: Having different alleles for a given trait; Aa, Yy Dominant: An allele that is expressed in the phenotype. Use capital letters Recessive: An allele that is not expressed in the phenotype because it is masked by the dominant allele. Use lowercase letters YY = pure breeding yellow dominant
yy= pure breeding green recessive Y Y
y Yy Yy
y Yy Yy P= parent generation F1 generation F2 generation 3 yellow to 1 green ratio for phenotype
1:2:1 ratio for genotype Mendel selfed the F1 generation and this is the ratio from that cross Homozygous means that the alleles are identical so..... We will assign the yellow pea with the genotype YY and the green pea with the genotype yy Y Y
y Yy Yy
y Yy Yy Since the F1 generation did not have any green peas, we can deduce that the Yellow pea must be dominant to the green pea How to construct a Punnett Square 2. Take the genotypes of the cross and place one pair of alleles at the top and the other pair at the side. This example will be one homozygous dominant and one heterozygous. 1. Draw a square box and place a cross inside the box to divide the box into four little boxes. A A
A AA AA
a Aa Aa If two people who are both carriers for a genetically inherited fatal recessive disease decide to become parents, what will be the odds that their children will also be carriers? What does it mean to be a carrier? Normal would be AA in this case since having the disease is recessive, aa
Being a carrier means that you carry one of the recessive allele which means that you are heterozygous, Aa A a
A AA Aa
a Aa aa 2 out of 4 or 50% will be carriers If a woman is homozygous normal and her husband is heterozygous for a genetically inherited recessive disease and they decide to become parents, what is the probability that they will have a healthy child? Woman is homozygous and normal so her genotype is AA
Man is heterozygous so he is a carrier and his genotype is Aa A A
A AA AA
a Aa Aa
This is a recessive disease so the child would need to inherit two recessive allele to get the disease.
Based on the Punnett square, 4 out of 4 will be healthy or 100% If two parents are heterozygous for a genetically inherited dominant trait, what is the probability that they will have a child together who has this trait in his or her phenotype? Both parents are heterozygous meaning that they will have two different alleles, Aa A a
A AA Aa
a Aa aa This is a dominant trait so if one allele is present then the person will have this phenotype.
In this scenario, the child will have a 3 out of 4 or 75% chance of having this phenotype. If two parents are homozygous for a genetically inherited recessive trait, what is the probability that they will have a child who does not have this trait in his or her phenotype? Both parents are homozygous recessive, aa a a
a aa aa
a aa aa This means that there is a 0% chance that they will have a child that does not have this phenotype or 100% chance of a child having this phenotype. (aa means that the child has the phenotype) Alleles of a gene determine the phenotype (What it looks like) and genotype (actual alleles) Alleles can be dominant (overpowering) or recessive (hidden when with a dominant allele) Alleles can be homozygous (identical) or heterozygous (different)