Download Unit 7 Heredity: Chp 11 Mendelian Genetics Notes

INTRODUCTION TO GENETICS
Do you look like your parents?
People noticed family resemblances throughout time
No until mid 1800’s that a scientific study carried out
Resemblances among pea plants was first study
Austrian Monk Gregor Mendel
Transfer of physical characteristics from parents to offspring lies behind the microscopic
level of sex cells and bits of heredity material within them called chromosomes.
MEDEL’S LAWS OF HEREDITY
Mid 19th Century
Austrian Monastery
Gregor Mendel
WHY MENDEL SUCCEEDED
First important studies of heredity
Heredity = passing on of characteristics from parent to offspring
Carefull study of genetics required for inheritance
Genetics = the branch of biology that studies heredity
Traits = characteristics that are inherited
Mendel first to predict how traits are transferred from one generation to the next
Mendel studied garden pea plants in his experiment
Reproduces sexually
2 distinct sex cells = male and female
Gametes = sex cells
Both female and male gametes are in the same pea flower
Male gamete = pollen grain
Female = ovule
Fertilization = uniting the male and female gametes
Pollen grain fuses with the egg in the ovule
Ovule will mature into a seed
Mendel made a cross
Transferred pollen grain from one plant to another with different traits
The pea plants structure of flower made the identity relatively easy
He studied one trait at a time to control the variables
Analyzed his data mathematically
Tall Pea = from population of plants been tall for generations and always produced tall
offspring.
True breeding for tallness.
Short Pea = true breeding for shortness
MENDEL’S MONOHYBRID CROSSES
What did he do with the Tale and Short pea plants?
Crossed them to produce new plants
1st experiments were Monohybrid Crosses
2 parent plants differed by a single trait = Height
Generations
First Generation:
- Selected 6 foot tall pea plants(true breeding)
- Cross pollinated them with short pea plants less than 2 foot tall
- All offspring in first generation were 6 foot tall
- As if the shorter parent never existed!
Second Generation:
- Crossed 2 tall offspring plants from F1
- Let them self pollinate
- ¾ of offspring were tall
- ¼ were short
- Fig 12.2
- As if short parent reappeared!
P1 generation = The original parents, true breed tall and short peas (P= parent)
F1 generation = the offspring of parents (F = filial son/daughter)
F2 generation = Offspring of cross of F1 plants
Alleles
Mendel did monohybrid crosses with a total of 7 pairs of traits
Fig 12.3
Every case he found that one trait seemed to disappear in F1 generation, only to reappear
in F2 generation
He concluded that each organism has 2 factors for each of its traits
We now know these factors are genes located on chromosomes
Genes exist in alternative forms
Alleles = different gene forms
Gene = a segment of DNA located on the chromosomes
Example: each of Mendel’s pea plants had 2 alleles that determined its height
A plant could have 2 alleles for tallness
2 alleles for shortness
1 allele for tallness and 1 allele for shortness
Alleles are located on different copies of a chromosome
One is inherited from the female parent and one from the male parent.
Dominant and Recessive
F1 offspring were all tall even though one parent was short
Only one trait was observed
Dominant trait = one observed
Recessive trait = one that disappeared
Allele for tall plant = Dominant
Allele for short plant = Recessive
Pea plants with 2 alleles for tall = Tall
2 alleles for short = Short
1 tall and 1 short = Tall due to dominant
Allele for short plants is recessive to allele for tall plants
1st Law of Heredity
Cross of F1 tall plants = shortness reappeared
To explain this, Mendel made 1st law of heredity
Law of Segregation = the 2 alleles for each trait must separate when gametes are
formed. A parent therefore passes on at random only one allele for each trait to each
offspring
PHENOTYPE AND GENOTYPES
P1 original true breeding tall plants = offspring all tall
F1 tall plants yielded both tall and short offspring.
2 organisms can look alike but have different underlying gene combinations.
Phenotype = the way an organism looks
Phenotype of tall plants = tall
Genotype = the gene combination an organism contains
TT = genotype for 2 alleles for tallness
Tt = genotype for 1 tall and 1 short
Phenotypes look alike
Can’t always know an organisms genotype simply by looking at its phenotype
Homozygous = if 2 alleles for the trait are the same
Homozygous Dominant:
TT = tall
Homozygous Recessive:
tt = short
Heterozygous = if 2 alleles for the trait are different
Tt = heterozygous (tall)
MENDEL’S DIHYBRID CROSSES
Used another set of crosses in which he used peas that differed in 2 traits rather than just
one.
Dihybrid Cross = crosses involving 2 different traits
True breeding pea plants had Round Yellow seeds (RRYY)
He crossed these with Wrinkled Green seeds (rryy)
Round trait was dominant
Yellow trait was dominant
F1 plants all had Round Yellow seeds
Fig 12-6
F2 some Round Yellow seeds and others Wrinkled Green
Also, Round Green and Wrinkled Yellow
He sorted and counted F2 generations = Ratio
9 Round Yellow: 3 Round Green: 3 Wrinkled Yellow: 1 Wrinkled Green
2nd Law of Heredity
Used to explain the dihybrid results
Law of Independent Assortment = genes for different traits are inherited independently
of each other.
When Pea plants with genotypes RrYy produce gametes, the alleles R and r will separate
from each other (law of segregation) as well as from the Y and y (law of assortment).
These alleles can then recombine in 4 different ways.
PUNNETT SQUARES
1905 Reginald Punnett (English biologist)
Punnett Square = shorthand way of finding the expected proportions of possible
genotypes of offspring
If you know the genotypes of the parents, you can use a Punnett Square to predict the
genotypes of the offspring.
1. Monohybrid Crosses
- F1 tall pea plants each is Tt
- ½ gametes of each parent contain T allele
- other ½ would contain t allele
- 2 boxes tall and 2 wide = 4 total
- Each parent can produce 2 kinds of gametes for the trait
- After genotypes determined you can determine Phenotypes
- Fig 12.7
- ¾ offspring are tall
- ¼ are short
- Genotype= 1 TT: 2 Tt: 1 tt
- Phenotype = 3 tall: 1 short
2. Dihybrid Crosses
- 2 traits are considered (shape and color)
- All F1 plants were Round and Yellow and heterozygous for each trait (RrYy)
- Seed color and shape inherited independently of each other
- Each F1 plant will produce gametes containing the following combinations of
genes with equal frequency:
- Round Yellow (RY)
- Round Green (Ry)
- Wrinkled Yellow (rY)
-
Wrinkled Green (ry)
- 4 boxes on each side
- 16 boxes total
- 9 RY: 3 Ry: 3 rY: 1 ry