genetics
Post on 24-Feb-2016
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Genetics The scientific study of
heredity.
What is Heredity ?
• The passing of traits from parents to their offspring
1860s: Gregor Mendel
• “Father of Genetics” (my favorite
Austrian monk)
• He figured out how traits are passed on before we knew about DNA, or chromosomes or meiosis
Mendel studied heredity with pea plants• Why pea plants? (was it luck?)– 1. Small– 2. Easy to grow– 3. Produces large numbers of offspring– 4. Matures quickly– 5. Plants can self or cross fertilize– 6. Many varieties– 7. Each trait has two forms• Tall/short purple/white flowers• green/yellow seeds colored/white seed
coat• green/yellow seed pods inflated/narrowed
seed pod• wrinkled/smooth seed
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
Genotype:
• Genotype is the combination of alleles for a particular expressed with letters
• Homozygous dominant:– TT - two capital letters
• Heterozygous– Tt - one of each letter (big and small)
• Homozygous recessive– tt - two lower case letters
Phenotype:• Phenotype is physical expression of the
genotype 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 : 1tt
Phenotypes: 3 Tall: 1 Short.
VocabularyHeredityLaw of DominanceLaw of SegregationLaw of Independent AssortmentAlleleGenotypePhenotype
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