1. Mendel’s Studies of Traits
Chapter 8
Section 1 The Origins of Genetics
Mendel’s Studies of Traits
The passing of traits from parents to offspring is called heredity.
Gregor Mendel is considered to be the father of genetics.
The patterns that Mendel discovered form
the basis of genetics, the scientific study
of heredity.

2. Parental generation- first pair of plants or animals in a study
Chapter 8
3 Principles of Genetics
Parental generation- first pair of plants or animals in a study
First Filial generation (F1)— the results of the parental cross
Second Filial generation (F2)- offspring produced by the first filial generation.
Chromosomes(found in nucleus)-> DNA->Genes

3. Mendelian Theory of Genetics
Chapter 8
Section 1 The Origins of Genetics
Mendelian Theory of Genetics
For each inherited trait, an individual has two copies of the gene, one from each parent.
** It takes at least two genes to express a trait **
There are alternative versions of genes. The term allele refers to either member of a pair of genes that expresses a trait. Ex. Eye color, hair color, textures, shape, etc.

4. Mendelian Theory of Genetics, contd.
Chapter 8
Section 1 The Origins of Genetics
Mendelian Theory of Genetics, contd.
3. Principle of Dominance - states that one allele (or gene) in a pair may prevent the other allele (or gene) in the pair from being expressed.
A gene that masks or over-rides another gene in the pair is called dominant. The hidden or repressed gene is called recessive.
Dominant genes-capital letters.
Recessive genes-lowercase letters.
2 gene pairs are identical-TT or tt –homozygous
Both alleles are dominant (TT)-homozygous dominant
Both alleles are recessive (tt)-homozygous recessive

5. Chapter 8 Mendelian Theory
Section 1 The Origins of Genetics
Mendelian Theory
When 1 allele is dominant and the other recessive (Tt) it is called heterozygous.
Genotype-pair of alleles that make up a trait. Ex: TT, tt. or Tt.
Phenotype-what is actually seen or expressed physically in the organisms appearance.
Law of Segregation- states that members of each pair of genes separate when gametes are formed.
Law of Independent Assortment – states that the alleles of different genes (for traits) separated independently of one another during gamete formation.

6. In squash, yellow color is dominant over white color
In squash, yellow color is dominant over white color. Determine the possible genotypes and phenotypes when a heterozygous yellow squash is crossed with a homozygous white squash.

7. Chapter 8
Section 1 The Origins of Genetics
Punnett square – named for Sir Reginald Punnett (British geneticist), helps you visualize probable results of genetic crosses.
Symbols for all possible gametes from the male goes across the top of the grid.
Symbols for all possible gametes from female appear along the left side.
Monohybrid cross- a cross involving one trait (one pair of alleles)
Dihybrid cross- a cross involving 2 traits and therefore at least 4 alleles.

8. The polled or hornless condition in cattle (P) is dominant over the horned (p). A cross between homozygous polled and horned cattle would yield polled offspring.
What are the genotypes and phenotypes for the F1 and F2 generations?

9. Dihybrid Cross
In watermelons, solid green is dominant to striped. Short melons are dominant to long melons. What would be the genotypes and phenotypes if a homozygous green short melon was crossed with a homozygous striped long melon?
If two of the F1 melons were crossed what would be the resulting genotypes and phenotypes?

10. Complex Patterns of Inheritance
Incomplete Dominance- both alleles of a pair for a trait are expressed because neither one is completely dominant over the other. It produces an intermediate (middle) phenotype between the two contrasting ones.
No alleles are recessive. A super script is used to represent incomplete dominance.

11. The genes for flower color in snap dragons plants show incomplete dominance. There are genes for red flowers and for yellow flowers but there are three phenotypes: red, yellow, orange. Give the genotypes and phenotypes of the F1 generation when a red flower is crossed with an orange flower.

12. Co-Dominance- when two dominant alleles are expressed at the same time, both forms of the trait are displayed. Co-dominance is different from incomplete dominance because both traits are displayed. (while in incomplete dominance there is a blend of the trait.
Ex: If you cross a homozygous red horse with a homozygous white horse the result is a heterozygous horse that has both red and white hair in equal numbers. This mixed color is roan.

13. Multiple alleles- Several human traits are determined by genes with multiple alleles, which are 3 or more alleles of a gene rather than only 2 for a trait. A person can have at most 2 of the alleles for a characteristic, one from each parent (but there exist more alleles for the characteristic).
Ex: Blood Type

14. Blood type is determined by three different alleles
Blood type is determined by three different alleles. Antigens on the cell membrane of the RBC causes clumping. The presence or absence of these antigens determines 4 different blood types.
Blood Types A and B show codominance.
Both A and B are dominant over O.
Type AB is the universal recipient.
Type O is the universal donor.

15. Rh Factor
An antigen found on the RBC. Received its name because it was first isolated in the blood of the rhesus monkey. Rh factor is inherited. A person whose RBC have the Rh factor are said to be “Rh positive”. If RBC do not have the Rh factor, the person is “Rh negative”.

16. 4. Polygenic inheritance- (continuous variation) traits that are determined by the interaction of genes at several different loci (different places on a single chromosome). The characteristics cannot be categorized as either/or, but the variation of phenotype can be classified as how much or what amount. Ex: skin color, intelligence, height, weight, etc.

17. Sex-Linked Traits
Sex in humans is determined by the X and Y chromosomes.
Sex linked traits in humans are determined by genes that are carried only on the X chromosome with no apparent allele on the Y chromosome.
Ex: Hemophilia, color blindness, duchanes muscular dystropy

18. Sex-Linked Problems
The normal daughter of a man with hemophilia (recessive sex-linked ) marries a man who is normal for the trait. What is the probability that a daughter will be a hemophiliac? A son?
Hemophilia is sex-linked in humans. Could a normal man and a woman who carried the gene for the disease, but does not have it, have a daughter with hemophilia?
Red-green colorblindness is caused by a recessive sex-linked allele. A color-blind man marries a woman with normal vision whose father was colorblind. What is the probability that they will have a colorblind daughter?
Two normal visioned parents produce a son who is colorblind. What are the genotypes of the parents?
A girl of normal vision whose father was colorblind marries a colorblind male. What will be the nature of their children with respect to colorblindness?

19. Sex-Linked Problems
A woman that is a carrier of hemophilia marries a hemophiliac man. What is the probability that their first child will be a hemophiliac?
A hemophiliac woman has a mother who is phenotypically normal. What are the genotypes of her mother and father?
What is the probability that a normal vision woman who marries a man who is colorblind, will have a daughter who is colorblind?
A phenotypically normal man who has a brother with muscular dystrophy marries a homozygous normal woman. What is the probability that any of their children will have muscular dystrophy?