Mendelian Patterns of Inheritance

Chapter 11

11.1 Gregor Mendel

Known as "The Father of Genetics" for his experiments on pea plants which established the basic rules of heredity.

In 1865, Mendel published his findings in a paper called Experiments on Plant Hybridization, which was mostly ignored at the time due to a number of reasons. First, Mendel was not well known in scientific community. Second, his theory ran against the popular model of blended inheritance.

Funfact: Mendel originally wanted to breed mice, but wasn't allowed to because it was considered scandalous

Mendel found that pea plants had a variety of traits, such as: stem length, pod shape, seed shape, seed color…. 

In 1865, Mendel published his findings in a paper called Experiments on Plant Hybridization, which was mostly ignored at the time due to a number of reasons. First, Mendel was not well known in scientific community. Second, his theory ran against the popular model of blended inheritance.

11.2 Mendel's Law of Segregation

Because the purple flower trait completely masks the white flower trait when true-breeding plants are crossed, the purple flower trait is called dominant, and the white flower trait is called recessive.

It didn’t matter what trait he studied either….

 Two copies of same allele = homozygous. Homo means "the same"  PP or pp

Some purple-flowered plants could be Pp.  Individuals that are purple, but had a white parent, are heterozygous: Pp. Hetero means "different".

The F1 cross

Pp  x Pp   purple x purple

Review Terms

F1  vs   F2

True Breeding vs Hybrid

Self Pollination vs  Cross Pollination

Homozygous vs Heterozygous

The physical appearance of an organism is its phenotype. Purple-flowered would be a phenotype.

The actual composition of the organism's alleles for a gene is its genotype: Pp is a genotype.

Organisms have many different genes some have thousands, and complex organisms have 10 times that number.

GENOTYPE PHENOTYPE

          Pp        purple flowers

 rr wrinkled seeds

TT tall

tt short

Practice with Punnett Squares

1.  A  round seeded plant (RR) is crossed with a wrinkle seeded plant (rr).  What are the phenotypes of the offspring?

2.  Two heterozygous purple flowered pea plants are crossed.  What are the phenotypes of their offspring and in what proportion?

3.  A plant with green seeds (yy) is crossed with a heterozygous plant.  What percentage of their offspring have yellow seeds?

The DIHYBRID cross

A dihybrid is an individual that is a double heterozygote

RrYy x RrYy

The outcome is always 9:3:3:1

9 for the two dominant traits

Round and Yellow

3’s are for offspring that have one dominant and one recessive:

3 are round, green3 are wrinkled, yellow

1 is for the two recessive traits:

1 is wrinkled, green

It may be faster to solve problems mathematically. This one is NOT 9:3:3:1

Consider the cross between a plant with round seeds, purple flowers to one with wrinkled seeds and white flowers ...

RrPp x rrpp

What does INDEPENDENT ASSORTMENT mean?

- states that each trait is separate, the alleles do not influence each other. This is why the 9:3:3:1 ratio is constant.

Example: Pod shape does not affect pod color.

Mendel would have never developed this law if he'd chosen traits located on the same chromosome. Why do you think that would have altered his results?

Genes located on the same chromosome do influence each other.

A note about probability

* It doesn't matter how often you flipped a coin or how many times it's already shown heads, the probability is ALWAYS 50% of heads/tails.

The gambler's fallacy is a logical fallacy where people gambling believe that a losing streak will turn around.

Example: I've tossed a coin 4 times, all four times it came up heads. What is the probability that my next toss will be heads?