Download Genetics notes

1/3/2016
Genetics
The study of heredity
Gregor Mendel
Terminology
Probability
Punnett Squares
Gregor Mendel
Central Questions
• Who was Gregor Mendel and what did he
contribute to genetics?
• What is meant by dominant and
recessive?
• What are the meanings and relationships
of traits, genes and alleles?
• What are the meanings and relationships
of genotype and phenotype?
Mendel’s Experiments
• 1860’s Austrian Monk
• Studied inheritance of traits in pea
plants
• Used his math background to
make new hypotheses about
inheritance.
• Known as the “Father of Genetics”
• Mendel was surprised to find that the
traits didn’t blend. The offspring
showed one trait or the other.
–Dominant - more powerful, covers
the recessive; written as a capital
letter
–Recessive - weaker, only shows if
it isn’t overpowered by a dominant
trait; lower case letter
Summary of Mendel’s Work
• Genetics is the study of heredity.
• Heredity is controlled by units
called genes. One gene comes
from each parent
• If two forms of the gene are
present, only the dominant one
shows (Law of Dominance)
1
1/3/2016
Summary of Mendel’s Work
• The two versions of the gene are
separated when gametes are
made. The organism only passes
down one of it’s genes. (Law of
Segregation)
• Genes for different traits are
passed down independently of the
others (Law of Independent
Assortment)
• Each parent gives the offspring one of
each chromosome
• Therefore the organism has two
copies of each gene, one from each
parent
• Each gene can be the dominant allele
or the recessive allele, depending on
what is inherited from the parent
• The organism can have two
dominants, two recessives or one of
each
Phenotype
• Only one allele appears in
the organism (The flower
is either purple or white)
• We say that version of the
gene is ‘expressed’
• The phenotype describes
what allele of the trait is
being expressed (the
phenotype of the top
three flowers is purple,
the bottom is white)
Traits, Genes and Alleles
• The trait is a characteristic of the
organism
–Examples: height, flower color, pea
coat
• Genes are what control the traits. They
are the unit of inheritance
• An allele is a version of the gene
–Examples of alleles for plant height
would be the tall allele and the short
(dwarf) allele
Genotype
• The genotype is the genetic makeup of the
organism (what alleles it has; the letters)
– Homozygous- Both genes for the trait are
the same allele. Can be homozygous
dominant (TT) or homozygous recessive
(tt)
– Homozygous is also called pure-bred or
true-breeding
– Heterozygous- Organism has a dominant
allele and a recessive allele (Tt)
– Heterozygous is also called hybrid
Parent Crosses
If we know the genotypes of two parents we can predict
the likely genotypes of their offspring
The steps of genetic cross experiments:
1. Select letters for each allele and list
them
• T=tall t= short
2. Write the Parent Cross
• TT x tt
3. Fill in the Punnett Square
4. List the predicted genotype and
phenotype probabilities for the offspring
2
1/3/2016
Punnett Squares
• Always write the capital letter first
• Each possible gene for one parent is put above
a square on top, the other on the side (it doesn’t
matter which one goes where)
• The square is filled down from each letter on
the top and across for the letters on the side.
• Only two letters for a trait appear in each box.
• Each box represents the percentage of
offspring with that genotype.
Generations
• P Generation- Parent generation
• F1 Generation- First generation of
offspring
• F2 Generation- Second
generation of offspring (with F1 as
the parents)
Central Questions
• Who was Gregor Mendel and what did he
contribute to genetics?
• What is meant by dominant and
recessive?
• What are the meanings and relationships
of traits, genes and alleles?
• What are the meanings and relationships
of genotype and phenotype?
• How are Punnett Squares used to predict
the offspring of a parent cross?
3