Mendelian Genetics

My AP Biology Thoughts Unit 5 Heredity

Welcome to My AP Biology Thoughts podcast, my name is Adrienne Li and I am your host for episode #99 called Unit 5 Heredity: Mendelian Genetics. Today we will be discussing Mendel’s experiment with pea plants, and the three laws he proposed from his results.

Segment 1: Introduction to Mendelian Genetics

• First off, lets define what mendelian genetics is. It’s a concept about heredity that was formed by Gregor Mendel and states that genes are distinct units that come in pairs and are inherited independently from other genes. These make up an offspring’s genome and provide the basis of inheritance for sexual reproduction. Now, Mendel didn’t just come up with this concept randomly. It started with an experiment he conducted where he crossbred pure pea plants with different traits such as purple vs. white flowers and tall vs. short stems. The F1 generation all carried the same trait as one of the parents, which refuted the prior notion that inheritance was a blend of the parent’s traits since the flowers weren’t pink. What was more strange was the F2 generation, which had a 3:1 ratio where 3 individuals had the same trait as one parent and 1 individual had the other parent’s trait.

Segment 2: More About Mendelian Genetics

• From these experimental results, Mendel summarized his findings in 3 laws. The first one is the law of dominance where genes have two alleles, and the dominant allele will conceal the recessive allele. This was seen in the F1 generation where all the flowers were purple even though one of the parents had white flowers. This meant that purple was dominant while white was recessive, thus only the purple flower trait was expressed. The F2 generation with a 3:1 ratio further confirmed this concept because breeding heterozygotes results in 4 offspring, one that is homozygous dominant, two that are heterozygous, and one that is homozygous recessive which means 3 individuals will express the dominant trait and one will express the recessive trait. Next, the second law is the law of independent assortment which states that alleles of two or more different genes are sorted into gametes independently of another where the allele a gamete receives for one gene does not influence the allele receive for another. This idea was demonstrated when he performed dihybrid crosses which tested two different traits and it resulted in a 9:3:3:1 ratio. This showed that traits such as flower color and stem length are inherited independently from each other, and one does not influence how the other trait is inherited. Lastly, the law of segregation says that during reproduction, the gametes only receive one copy of a gene from each parent at random. This was demonstrated by his 3:1 ratio where during segregation in meiosis of the F1 generation, each gamete acquired one of the two genes so that there were three possible combinations, either homozygous dominant, heterozygous, or homozygous recessive. Since there were two ways to form heterozygotes, either receiving one dominant and one recessive allele from either parent, and because heterozygotes and homozygous dominant gametes express the same phenotype, it supported the 3:1 ratio and law of segregation.

Segment 3: Connection to the Course

• Tying this back to the overall ideas in Unit 5, mendelian genetics corroborates the...