Genetics- the Science of Heredity and how traits are inherited
Heredity- The passing of traits from parents to offspring
Dominant- When one trait overpowers or hides the other trait.
Recessive- The trait that is hidden or masked
Genes- Specific traits are carried as genes on chromosomes. Genes are segments of DNA.
Alleles- The alternate forms of a gene. You receive one allele from each parent and have 2 alleles to describe a trait. Dominant: A Recessive: a
Genotype- How the Two Alleles are read on the gene. Ex. Aa
Homozygous dominant- When both alleles are Dominant: AA. Only the dominant trait is seen.
Homozygous recessive- When both alleles are Recessive: aa. Only the recessive trait is seen.
Heterozygous- When one allele is Dominant and the other is Recessive: Aa. The Dominant trait covers up the Recessive trait.
Phenotype- The physical appearance of the trait. Ex. Brown eyes
Mendel- 1866. The Father of Genetics. Worked with pea plants to study heredity.
Mendel’s First Set of Experiments:
Mendel ﬁrst experimented with just one characteristic of a pea plant at a time. He began with ﬂower color. The parent plants in the experiments are referred to as the P (for parent) generation. F1 and F2 Generations: The offspring of the P generation are called the F1 (for ﬁlial, or “offspring”) generation. All of the plants in the F1 generation had purple ﬂowers. Mendel wondered what had happened to the white-ﬂower characteristic. Mendel allowed the F1 generation plants to self-pollinate. Some of the F2 generation plants had white ﬂowers. He studied F2 generation plants, and for every three purple-ﬂowered plants, there was an average of one white-ﬂowered plant. Law of Segregation: Mendel did the same experiment for all seven characteristics. In each case, one value of the characteristic disappeared in the F1 plants and then showed up again in the F2 plants. And in each case, 75 percent of F2 plants had one value of the characteristic and 25 percent had the other value. Based on these observations, Mendel formulated his ﬁrst law of inheritance. This law is called the law of segregation. It states that there are two factors controlling a given characteristic, one of which dominates the other, and these factors separate and go to different gametes when a parent reproduces.
Mendel’s Second Set of Experiments:
Mendel next investigated two characteristics at a time. For example, he crossed plants with yellow round seeds and plants with green wrinkled seeds. The results of this cross, which is a dihybrid cross. F1 and F2 Generations: Mendel observed that plants in the F1 generation were all alike. All of them had yellow and round seeds like one of the two parents. When the F1 generation plants self-pollinated, however, their offspring F2 generation all possible combinations of the two characteristics. These combinations of characteristics were not present in the F1 or P generations. Law of Independent Assortment: Mendel repeated this experiment with other combinations of characteristics, such as ﬂower color and stem length. The results of Mendel’s second set of experiments led to Mendel’s second law. This is the law of independent assortment. It states that factors controlling different characteristics are inherited independently of each other.
Punnett square- Shows cross between the parents for a particular trait with all possible combinations of genotype and phenotype.
Possible Genotypes: 50%- Bb 50%- bb
Possible Phenotypes: 50%- Brown eyes 50%- Blue eyes
Monohybrid Cross- A Punnett square crossing only one trait. See figure right.
Dihybrid Cross- A Punnett square crossing 2 traits. See figure right.
Multiple Alleles- some traits have multiple alleles such as blood types: i, IB, IA
Incomplete Dominance- When there are two alleles that are neither dominant nor recessive, both are expressed. This is seen in 4 O’clock Flowers.