Chapter 10 2

Chapter 10.2 Mendelian Genetics NWRC Bio 30

Gregor Mendel • An Austrian Monk who did groundbreaking work into the theories of heredity. According to the textbook his work was published in 1866.

Mendel • Mendel discovered some basic laws which governed the passage of a trait from one member of a species to another member of the same species. • Demo Mendel

Mendel • Why Mendel studied pea plants • This site gives us the background on how plants self-fertilize and how they can be cross-fertilized as well

vocabulary terms • GENOTYPE = the genes present in the DNA of an organism. We use a pair of letters (ex: Tt or YY or ss, etc.) to represent genotypes for one particular trait. There are always two letters in the genotype because (as a result of sexual reproduction) one code for the trait comes from mom & the other comes from dad, so every offspring gets two codes (two letters).

Mendel • Suppose one of your parents had the genotype AABB then you would have inherited AB from this parent. Suppose also that your other parent had the genotype aabb then you would have inherited ab from this parent. The combinations of AB and ab are parental types. Your genotype is AaBb and some of your children will inherit these parental types either AB or ab from you. However, it is also possible for some of your children to inherit new combinations called "recombinants from you. These are Ab and aB.

What does that mean in real terms? • Lets say A is a tallness allele so a would be shortness • B is brown eyes and b is blue eyed • one of your parents had the genotype AABB so that parent (we’ll call him dad) is TALL and Brown eyed • your other parent (called Mom) had the genotype aabb (Short and blue eyed) • Your genotype would be AaBb • What would you look like???

What does that mean in real terms? • Your genotype would be AaBb • So you would have the dominant trait for tallness and the dominant trait for brown eyes.

vocabulary terms • When we have two capital or two lowercase letters in the GENOTYPE (ex: TT or tt) it's called HOMOZYGOUS ("homo" means "the same"). Sometimes the term "PURE" is used instead of homozygous. • When the GENOTYPE is made up of one capital letter & one lowercase letter (ex: Tt) it's called HETEROZYGOUS ("hetero" means "other"). A heterozygous genotype can also be referred to as HYBRID.

vocabulary terms • PHENOTYPE = how the trait physically shows-up in the organism. The simplest way to determine an organism's phenotype is to just look at it. Examples of phenotypes: blue eyes, brown fur, striped fruit, yellow flowers. • ALLELES = • Alleles are alternative forms of the same gene. Alleles for a trait are located at corresponding positions on homologous chromosomes.

vocabulary terms • PHENOTYPE = how the trait physically shows-up in the organism. The simplest way to determine an organism's phenotype is to just look at it. Examples of phenotypes: blue eyes, brown fur, striped fruit, yellow flowers. • ALLELES = • Alleles are alternative forms of the same gene. Alleles for a trait are located at corresponding positions on homologous chromosomes.

Mendel’s Pea Plants • Monks have a lot of free time and Mendel spent his time crossing pea plants. As he did this over & over & over & over & over again, he noticed some patterns to the inheritance of traits from one set of pea plants to the next. By carefully analyzing his pea plant numbers he formed his hypotheses regarding genetics.

The original cross for the tall & short pea plants was:

parents

F1 Offspring

Genotype

TT X tt

Tt -100%

Phenotype

Tall x Short Tall – 100%

Mendel’s Pea Plants •

While Mendel was crossing (reproducing) his pea plants (over & over & over again), he noticed something interesting. When he crossed pure tall plants with pure short plants, all the new pea plants (referred to as the F1 generation) were tall. Similarly, crossing pure yellow seeded pea plants and pure green seeded pea plants produced an F1 generation of all yellow seeded pea plants. The same was true for other pea traits:

Mendel’s Pea Plants • So, what he noticed was that when the parent plants had contrasting forms of a trait (tall vs short, green vs yellow, etc.) the phenotypes of the offspring resembled only one of the parent plants with respect to that trait.

Mendel’s Big Surprise! !!!!

• Then Mendel takes two of the "F1" generation (which are tall) & crosses them. I would think that he is figuring that he's going get all tall again (since tall is dominant). But for some reason he gets some short plants from this cross! His new batch of pea plants (the "F2" generation) is about 3/4 tall & 1/4 short.

The F2 Generation

Mendel My LAW of Segregation says that of a pair of characteristics only one can be represented in a gamete. What I mean was that for any pair of characteristics there is only one gene in a gamete even though there are two genes in ordinary cells.

What does that mean?

Mendel If your eyes are blue, green or grey you have two alleles for blue eyes (bb) You got one from your mom and one from your dad – everyone comes from 2 gametes (egg and sperm) so you may get 2 alleles but only one trait is expressed.

Monohybrid Cross • cross involved organisms that differed for a single character • examples in pea • tall x short, • purple x white, • round x wrinkled

Dihybrid Cross • A dihybrid cross is a cross between two F1 offspring of two individuals that differ in two traits examples in pea • tall x short, and • purple x white,

Example of another dihybrid cross • SsYy x SsYy Note both maternal and paternal alleles are heterozygous • The dihybrid cross was invented by Mendel to discover the independent assortment of alleles during gamete formation.

Mendel My next LAW is the law of "Independent Assortment". This says that for two characteristics the genes are inherited independently.

Mendel My third LAW – the Law of segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

Mendel! • Dominant Genes review

Review Questions • 1. Which cross would best illustrate Mendel's Law of Segregation? • A. TT x tt B. Hh x hh C. Bb x Bb • D. rr x rr • The right answer is C because - both parent show dominant trait, but some recessive offspring will be produced (each parent carries a "b")

Review Questions • 2. In the cross Yy x Yy, what percent of offspring would have the same phenotype as the parents? • A. 25% B. 50% C. 75% D. 100% • C is right - in the completed p-square, 3 of 4 boxes will have at least 1 "Y", producing the dominant phenotype (same as parents)

Review Questions • In a certain plant, purple flowers are dominant to red flowers. If the cross of two purple-flowered plants produces some some purple-flowered and some redflowered plants, what is the genotype of the parent plants? • A. PP x Pp B. Pp x Pp • C. pp x PP • D. pp x pp • answer B is right - for any offspring to be recessive, each parent MUST have at least one "p"

Chapter Questions 1.

Chapter Questions The Law of Segregation Goes like so: During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

2 Continued • The Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another. Mendel noticed during all his work that the height of the plant and the shape of the seeds and the color of the pods had no impact on one another. In other words, being tall didn't automatically mean the plants had to have green pods, nor did green pods have to be filled only with wrinkled seeds, the different traits seem to be inherited INDEPENDENTLY.

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