1. Genetics 4-3
Genetics 4-3
3 Minute Video: 18 things you should know about yourself!
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2. Genes – determine the characteristics of an organism.
Alleles – alternate forms of genes
Ex. Gene = seed color
Green or Yellow  Alleles
*Remember*
Only 1 allele for each trait is passed on to the offspring from each parent!

3. The Terms… Genotype – genetic make-up of an individual Phenotype
Ex. T T = genotype (letter combination)
Each letter is inherited from different parents.
Dad Mom
T T
Phenotype
Expression of an individual’s genes…what it looks like.
Ex. Tall = phenotype (physical appearance)

5. Genotype Possibilities
Homozygous (purebred)
An organism with genes that are alike.
Ex. BB or bb
Ex. Homozygous Dominant Genotype.
-TT …capital letters illustrate dominance
…lower case letters illustrate recessiveness
Heterozygous (hybrid)
An organism with genes that are different
Ex. Bb

6. Homozygous recessive genotype
Both alleles are the same
Both represented by lower case letters
Example = tt
Organisms with different genotypes may have the same phenotype
Ex. Tt = tall , TT = tall
Assume T = tall and t = short
**For an organism to show a recessive phenotype, the genotype must be homozygous recessive.
Example: tt = short

7. Mendel’s 1st Law Law of Segregation Law of Independent Assortment
Hereditary characteristics are determined by factors called genes.
For each characteristic, an offspring gets 2 genes, one from each parent.
The 2 genes of each pair segregate from each other and end up in separate gametes.
Law of Independent Assortment
A) Traits are inherited independently of each other.

8. Solving Genetics Problems
If you know the genotype of the parents, it is possible to predict the likelihood of an offspring inheriting a particular phenotype.
- Probability – the chance that a given event will occur
- Punnett Square – named for R.C. Punnett, British biologist and mathematician
- Probability is shown through monohybrid crosses – genetic cross involving only 1 trait of the parents.

9. Examples…
Heterozygous long hair female with homozygous short hair male (long is dominant). L l x l l
Homozygous tall plant (TT)
w/homozygous short plant (tt).
100% Tt 100% Tall
heterozygous tall
50% Ll = long hair % ll = short hair : 1 ratio (1Ll to 1ll)

10. PRACTICE If round seeds are dominant to wrinkled;
List the genotype of;
1. A homozygous round seed:
2. A heterozygous seed:
3. A wrinkled seed:
4. A purebred wrinkled seed:
5. A hybrid seed:
6. A purebred round seed: