Genetics The scientific study of heredity.
Dec 18, 2015
Genetics
The scientific study of heredity.
Vocabulary: Make sure to DEFINE these words as you go through the notes
Heredity PhenotypeVariation GenotypeDominant DNARecessive AllelePurebred MeiosisHybridHomozygousHeterozygous
What is Heredity ?
• The passing of traits from parents to their offspring
• Traits are passed to you through your parent’s sperm and egg– 50% of DNA from mom, 50% from dad
• Variation (differences in people’s traits) are due to the combination you inherit
1860s: Gregor Mendel• “Father of Genetics”
(he was an Austrian monk)
• He figured out how traits are passed on before we knew about DNA, or chromosomes or meiosis
To begin, Mendel created pure breeding plants• Purple
First Cross: pure purple with pure white• Cross fertilized P (Parental)
generation– Pure purple with pure white
– P Generation F1 Generation
purple white
X
All purple
Second Cross:Cross 2 F1 plants• Cross fertilized two of the offspring
of the F1 generation
purple
X
purple
white
purple purple
purple
Mendel’s Experiments led him to 3 Laws of Inheritance• The Law of Dominance
• The Law of Segregation
• The Law of Independent Assortment
The Law of Dominance:• Each individual has TWO factors for each trait known
as alleles– Some alleles dominate over others – the dominant
allele is expressed in the offspring
– Homologous chromosomes-same traits -different forms
Freckles
Dimples
Tongue roller
From Mom From Dad
No Freckles
No dimples
Non-roller
These are
alleles
Alleles are:
• Represented by a letter of the alphabet
Capital = dominant allele A
lowercase = recessive allele a
• Each allele corresponds to a gene on a chromosome- the alleles represent different forms of the gene
Law of Segregation– The two alleles for each trait separate
and move into different gametes – meiosis
– Since only one egg or one sperm will contribute to the new offspring, only one allele for a trait is passed on
– The chance that any allele will be passed on is 50%
Law of Independent Assortment• the inheritance of alleles for one
trait doesn’t affect the inheritance of alleles for another trait
- occurs in meiosis during metaphase 1
Meiosis: division of the sex cells
Genotype:
• Genotype is the combination of alleles for a trait expressed with letters
• Homozygous dominant (purebred)– TT - two capital letters
• Heterozygous (hybrid)– Tt - one of each letter (big and small)
• Homozygous recessive (purebred)– tt - two lower case letters
Phenotype:• Phenotype is physical expression of the
genotype. (how it looks) Genotype Phenotype TT tall plant Tt tall plant
*dominant allele always is expressed over the recessive allele
tt short plant *recessive trait is expressed only if there is no dominant gene
If R = red, and r = white
• What is the genotype of homozygous dominant?– RR
• What is the heterozygous genotype?– Rr
• What is the homozygous recessive genotype?– rr
If R=red and r =white
• What is the phenotype of of RR?– Red
• What is the phenotype of Rr?– Red
• What is the phenotype of rr?– White
You can determine phenotypes using a Punnet square.
Tt Tt
Tt Tt
If two parents are crossed (TT X tt), what are the resulting offspring?
T T
t
t
parent
parent
Genotypes: 4 Tt
Phenotypes: 4 Tall
This is the first or parent generation
Now try on your paper a cross of two of the offspring from the parent generation. This is called the first generation or the F1.
Tt
Tt
T
T
t
t
TT
tt
Genotypes: 1TT: 2Tt : 1ttPhenotypes: 3 Tall: 1 Short.