Genetics
Genetics- the study of heredity
Heredity- the passing of genes / characteristics from parent to offspring.
Gregor Mendel
The Father of “Modern Genetics” He was the first person to trace the characteristics of successive
generations of a living thing.
• Mendel raised over 28,000 pea plants between 1856 to 1863
• He analyzed seven characteristics: plant height, pod shape, pod color, flower position, seed
color, seed shape, and flower color
• He bred and cross bred these plants and recorded the outcomes
• His conclusions led to rules that predicted patterns of heredity…..
In one experiment, Mendel cross pollinated plants with smooth yellow peas
and wrinkled green peas (called the Parent or P generation).
All of the 1st generation
Plants (called the 1st Filial or F1 Generation) were yellow and
Smooth.
He determined that there are two kinds of traits: Dominant
and
• Dominant- the stronger expressed trait
• Recessive- the weaker non-expressed trait
Next Mendel cross pollinated all of the smooth-yellow pea Plants from the F1 generation. And much to his surprise….. The 2nd generation of offspring (2nd filial or F2 generation)
had some green and wrinkled pea plants. For every 3 yellow and smooth pea plants there was 1 green and wrinkled plant. A 3:1 ratio.
With these results he concluded that : For each inherited characteristic, an individual
has two genes. One from each parent.
He called each of these genes alleles.
Mendel’s
Mendel’s observations needed to be put into a form that was easy to understand. Today genetic scientists still use Mendel’s format.
-- Dominant Characteristics are represented with the capital letter that the characteristic starts with.
EX: Yellow was a dominant Pea color and was represented with a
Y
--Recessive Characteristics are represented with
the lower case letter that the dominant characteristic starts with.
Other Terms
• Genotype- the actual alleles/genes an organism has for a given trait
EX: Rr
One dominant Right Hand allele One recessive Left Hand allele
• Phenotype- The actual allele that is expressed (what trait is seen)
EX: Rr
This persons phenotype is right handed- it is the dominant trait
• Heterozygous-
if an individual has two
different alleles for a given trait.
EX:
Rr
Right hand is the
phenotype but they have two
different genes.
• Homozygous-
if an individual has the
same two alleles for a given trait.
EX:
rr
Mendel’s
Laws of Heredity
• Law of Independent
Assortment-when the chromosomes split into chromatids during meiosis the two alleles for a given trait separate.
Punnett Square
Reginald Punnett-(1875-1967)
developed a diagram that biologists use to determine the probability of an offspring having a particular genotype.
The Punnett Square shows every possible Combination when combining one
maternal (mother) allele and one paternal (father) allele.
Two Types of Punnett Squares We Will Do
Monohybrid cross- involves the cross of one trait ex: eye color
How to Use A Punnett Square
Now You Try…
Dominant: Right Handed
Recessive: Left Handed
• Mom is Homozygous recessive for handedness
Mom’s Genotype: Mom’s Phenotype: Dad is Heterozygous for
Handedness
Dad’s Genotype: Dad’s Phenotype:
• Test cross- this determines whether a characteristic of a plant or animal is homozygous dominant (pure bred) or heterozygous. The individual is crossed with a homozygous recessive individual.
The proof Is in the Squares…
- If the offspring all display the dominant phenotype the tested individual is homozygous dominant - If some of the offspring display the recessive phenotype the tested individual is heterozygous
The Rule of Multiplication
• We can use the rules of probability to solve complex genetics problems.
• If we crossed two organisms both having the genotype AaBbCc, what is the probability that an offspring of this cross will have the genotype aabbcc? Since each allele pair assorts independently, we can treat this trihybrid cross as three separate monohybrid crosses:
Aa X Aa: Probability of aa offspring = ________
Bb X Bb: Probability of bb offspring = ________
Cc X Cc: Probability of cc offspring = ________
Sample Problem
Sample Problem
The Addition Rule
The Addition Rule
• As we can see from our Punnett square, there are two ways in which F1 gametes can combine to produce heterozygous offspring.
• The dominant allele can come from the egg and the recessive allele from the sperm, or ...
the dominant allele can come from the sperm and the recessive allele can come from the egg.
Sample Problem
Sample Problem
• Thomas Hunt Morgan and his students began studying the genetics of the fruit fly, Drosophila melanogaster, in 1907
• Morgan noted wild-type, or normal, phenotypes that were common in the fly populations
• Traits alternative to the wild type are called mutant phenotypes
• In one experiment, Morgan mated male
flies with white eyes (mutant) with female
flies with red eyes (wild type)
– The F1 generation all had red eyes
– The F2 generation showed the classical 3:1 red:white ratio, but only males had white eyes
Inheritance of Traits
• Sex linked genes- genes that are located only
on the X or Y chromosomes in mammals.
ex: Hemophilia is only located on
the X chromosome- it stops the bloods ability to clot.
• X chromosomes have genes for many
characters unrelated to sex, whereas the Y
• X-linked genes follow specific patterns of inheritance
• For a recessive X-linked trait to be expressed
– A female needs two copies of the allele (homozygous) – A male needs only one copy of the allele
(hemizygous)
• Morgan also did experiments with fruit flies
that show how linkage affects inheritance
of two characters
• Morgan crossed flies that differed in traits
of body color and wing size
– He found that body color and wing size are usually inherited together in specific
combinations (parental phenotypes)
Recombination
Recombination
• Mendel observed that combinations of traits in some offspring differ from either parent
• Offspring with a phenotype matching one of the parental phenotypes are called parental types
• Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants
Recombination Frequency
Recombination Frequency
• Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome
• Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur
between them and therefore the higher the recombination frequency.
• Genes that are far apart on the same chromosome can have a recombination frequency near 50%
• Such genes are physically linked, but genetically unlinked, and behave as if found on different
