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Mendel & Genetics
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Mendel’s work with peas
Gregor Mendel was an Australian monk who did a series of experiments to show that traits do NOT occur purely at random. He recognized that traits occur in family pedigrees according to certain patterns of inheritance. While at the monastery Mendel did experiments with pea plants.
He did pea plant studies, and discovered some seeds were smooth others wrinkled, some were green others yellow, some tall others short.
All sorts of variations were observed. He would record and experiment over and over, and he came across some patterns:

He observed the variation in these plants.
Term to know:
Variation: the occurrence of an inherited trait that makes a person or thing
different from other members with in the same species.
TALL X TALL = 4 ALL TALL PLANTS
TALL X TALL= 3 TALL 1 SHORT
TALL X SHORT = ALL 4 TALL
SHORT X SHORT = ALL SHORT
Was there a pattern in these results that supported Mendel’s hypothesis... that the traits of offspring came from their parents??? or were the results random???
Cross pollination of plants with certain traits with plants having contrasting traits.
He determined which traits were inherited by offspring plants
Established mathematical ratios from the counts of the contrasting traits that were displayed. These ratios describe "genetic factors" that control traits in terms of probability. Mendel's "Factors" are now known as genes
Definition: Gene:
A unit of heredity; a discrete portion of a chromosome thought to be responsible for the production of a single type of “factor". The "factor" responsible for the inheritance of a genetic trait.
Gene-Chromosome Theory:
Definition: A theory of genetic inheritance that is based on current understanding of the relationships between the biochemical control of traits and the process of cell division.

Chromosomes/Chromatids:
(Remember meiosis & mitosis) the chromosome is made up of 2 chromatids
Alleles:
Definition: One pair of genes that exist at the same location on a pair of homologous chromosomes and cause parallel control over the same genetic trait.
SOME MAJOR GENETIC PATTERNS: Genes of certain pairs can have contrasting (different or opposite) effects on the same trait.
However, only one may be expressed, while the other is masked.
Dominant: The gene that is expressed
Recessive: The gene that is masked
In "dominance problems" dominant alleles are usually symbolized by a capital letter (A) while the recessive allele for the same trait is given the lower case (a) of the same letter. If the two inherited traits are alike (TT or tt) the combination is called: homozygous If the two inherited traits of this allelic pair are different ( Tt) this combination is called : heterozygous ( or hybrid)
Example:
Garden peas: tall plants vs short plant
T = tall allele t = short allele
In fertilization homologous chromosomes are separated in meiosis, and the fertilization of the egg brings together the union of 2 different chromosomes.
In this cases two alleles for every trait are inherited.
Phenotype: The physical appearance, (what you see!) expression of the genetic make-up of the offspring
Genotype: The genetic makeup (the genes!) of the offspring that determines the phenotype
So Mendel tried to cross his tall plants and see what would happen. Remember, each trait has 2 genes. Mom will donate HALF of 2 genes and Dad will donate HALF of 2 genes. When Mendel crossed this is what the cross pattern looked like:

Dad’s	Mom T	Mom T
T	TT	TT
T	TT	TT

all 4 offspring are HOMOzygous dominate

100% dominate

Next Mendel tried to cross 2 short plants and see what would happen. Remember, each trait has 2 genes. Mom will donate HALF of 2 genes and Dad will donate HALF of 2 genes. But this time the results were different! WHY?? When Mendel crossed this is what the cross pattern looked like:

Dad’s	mom: t	mom: t
t	tt	tt
t	tt	tt

all 4 offspring are homozygous Recessive

this time! They’re all SHORT!

100% recessive

Next Mendel tried to cross 2 tall plants and see what would happen. Remember, each trait has 2 genes. Mom will donate HALF of 2 genes and Dad will donate HALF of 2 genes. But this time the results were different! WHY?? When Mendel crossed this is what the cross pattern looked like:

Dad’s	mom: T	mom: t
T	TT	Tt
t	Tt	tt

3 offspring are dominate, but only 1 is homozygous (TT) and this time there’s a recessive or SHORT who’s also homozygous (tt) and then 50% are heterozygous or Tt.

Note that 3 of these offspring would show the phenotype of the dominant gene,(being tall!) even though their "genetic make-up is different, (TT, Tt) while only one would show the recessive trait (tt) and be short.