Transcript Genes & Heredity

Genes & Heredity
Mendelian Genetics
Gregor Mendel (1822 – 1884)
• Austrian monk
• father of genetics
• provided the basis for our
understanding of heredity
• in his time he had no
knowledge of:
1. DNA
2. chromosomes
3. meiosis
• experimented with garden
peas
Why study pea plants?
1. easy to grow
2. mature quickly
3. easy to pollinate
4. have contrasting traits
• ex. height
• seed colour
• seed shape
Mendel’s Experiments
Experiment 1- Part A
Monohybrid Cross: (one trait studied)
height
• crossed:
P generation
(parental)
purebred tall
F1
1st generation:
x
purebred short
Predicted? All medium height
Found?
All tall
Hypothesized? Short trait was lost
Mendel’s Experiments
Experiment 1- Part B
• crossed:
tall
x
tall
F2
2nd generation: Predicted? All tall
Found? 75% tall & 25% short
Hypothesized?
Short trait was not
lost, just covered up
by tall trait
Mendel’s Laws
Principle of Dominance:
For contrasting traits :
• a dominant trait can ‘mask’ the recessive trait
Principle of Segregation:
• Each offspring inherits two ‘factors’ (alleles) for
each trait (one on each homologous chromosome)
• these ‘factors’ (alleles) segregate during the
formation of sex cells
predicted
• i.e. each gamete has only one allele
meiosis
Definitions
gene:
• section of DNA that codes for one characteristic
• ex. height in pea plants
allele:
• variations of a gene
• ex. tall vs. short
• all alleles are represented by the same letter
dominant allele:
• codes for a trait that is expressed (visible)
• represented by a capital letter
ex. T (for tall)
recessive allele:
• codes for the trait that is not expressed (covered up)
• represented by a lower case letter ex. t (for short)
Definitions
genotype:
• the pair of alleles present in an organism
• TT or Tt or tt
homozygous genotype:
• both alleles are the same
TT homozygous dominant
tt homozygous recessive
heterozygous genotype:
• both alleles are different
Tt
phenotype:
• physical appearance of organism
• determined by the combination of alleles (genotype)
Definitions
heredity:
• the passing of traits from parents to offspring
Mendelian Genetics
Experiment 1: Part A
P generation: purebred tall
purebred short
TT
tt
genotype:
meiosis
possible alleles:
T
T
t
t
punnett square:
• used to predict all possible genotypes from a cross
alleles of 1 parent
alleles tes of 2nd
parent
F1
T
T
t
Tt
Tt
t
Tt
Tt
phenotype:
100% tall
genotype:
100% Tt
(heterozygous)
Mendelian Genetics
Experiment 1: Part B
F1 generation:
hybrid tall x hybrid tall
genotype:
Tt
Tt
meiosis
possible gametes:
T
t
T
t
F2
T
t
T
TT
Tt
t
Tt
t t
phenotypes:
75% tall
25% short
phenotypic ratio
3:1
dom:rec
genotypes:
25% (TT) homozygous dominant
50% (Tt) heterozygous
25% (tt) homozygous recessive
Mendelian Genetics
Practice Problem:
Predict the genotypes and phenotypes of the
following cross:
P generation
Tt
x
tt
phenotypes ?
tall
short
F1
T
t
Genotypes: ?
t
Phenotypes: ?
t
Mendelian Genetics
Practice Problem:
Predict the genotypes and phenotypes of the
following cross:
T t x
t t
T
F1
t
Genotypes: 50 % T t
50 % t t
t
Tt
tt
Phenotypes: 50 % tall
t
Tt
tt
50 % short
Homework
Handouts:
1. Genetic Problems ‘Monohybrid Crosses’ # 1-7
GENETICS PROBLEMS
MONOHYBRID CROSSES
1. a)
Gg
1. b)
GG
Gg
gg
GENETICS PROBLEMS
MONOHYBRID CROSSES
2.
a)
b)
c)
25 % will have blue eyes
GENETICS PROBLEMS
MONOHYBRID CROSSES
3.
a)
Yy
yy
b)
YY
Yy
yy
GENETICS PROBLEMS
MONOHYBRID CROSSES
4.
5.
GENETICS PROBLEMS
MONOHYBRID CROSSES
6.
7.