Mendelian Patterns of Inheritance
Dec 23, 2015
Gregor MendelGenetics is the
study of heredity.Mendel was an
Austrian monk who studied genetics in the 1860s using garden peas.◦Had no knowledge of
cells or chromosomes.◦Did not have a
microscope.
Mendel knew that…Each male
flower part produced pollen (sperm)
Each female flower part produced egg cells.
FertilizationFertilization: During sexual
reproduction, when sperm and egg cells join to produce a new cell.
Pea flowers are self-pollinating.◦The seeds produced from self-
pollination inherit all of their characteristics from the single “parent”.
Cross-PollinationMendel had true-
breeding plants, that when allowed to self-pollinate, would produce identical offspring to themselves
Cross Pollination◦Mendel was able to
produce seeds that had 2 different parents, each with different characteristics.
Genes and DominanceTrait = a specific characteristic
that varies from one individual to the next.◦Examples: Height, Eye Color, Skin
ColorMendel studied seven pea plant
traits, each with two contrasting characters.
He crossed plants with each of the seven contrasting characters and studied their offspring.
Theories of InheritanceTheories of inheritance in Mendel’s
time:◦Based on blending◦Parents of contrasting appearance produce
offspring of intermediate appearanceMendel’s findings were in contrast
with this◦He formulated the particulate theory of
inheritance◦Inheritance involves reshuffling of genes
from generation to generation
Mendel’s first Conclusion
Mendel's first conclusion was that biological inheritance is determined by factors that are passed from one generation to the next.Today, scientists call the factors that determine traits genes.
AllelesEach of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait.The different forms of a gene are called alleles.
Mendel’s Second ConclusionMendel’s second conclusion is called the principle of dominance.The principle of dominance states that some alleles are dominant and others are recessive.
Segregation
◦ Mendel crossed the F1 generation with itself to produce the F2 (second filial) generation.
◦ The traits controlled by recessive alleles reappeared in one fourth of the F2 plants.
Copyright Pearson Prentice Hall
P GenerationF1 Generation
Tall Tall Tall Tall Tall TallShort Short
F2 Generation
Segregation
Segregation
• Each individual has a pair of factors (alleles) for each trait
• The factors (alleles) segregate (separate) during gamete (sperm & egg) formation
• Each gamete contains only one factor (allele) from each pair
• Fertilization gives the offspring two factors for each trait
Modern View of GeneticsEach trait in a pea plant is controlled
by two alleles (alternate forms of a gene)
Dominant allele (capital letter) masks the expression of the recessive allele (lower-case)
Alleles occur on a homologous pair of chromosomes at a particular gene locus
◦ Homozygous = identical alleles (AA or aa)
◦ Heterozygous = different alleles (Aa)
A capital letter represents the dominant allele for tall.A lowercase letter represents the recessive allele for short.In this example, T = tallt = short
Genotype vs. Phenotype Genotype
◦ Refers to the two alleles an individual has for a specific trait
◦ If identical, genotype is homozygous
◦ If different, genotype is heterozygous
Phenotype
◦ Refers to the physical appearance of the individual
Punnett SquaresTable listing possible genotypes
resulting from a cross.
◦All possible sperm genotypes are lined up on one side
◦All possible egg genotypes are lined up on the other side
◦Every possible zygote genotypes are placed within the squares
Monohybrid CrossOnly looks at one trait.Punnett square contains 4 boxes.
Individuals with recessive phenotype always have the homozygous recessive genotype
However, individuals with dominant phenotype:
◦ May be homozygous dominant, or
◦ Heterozygous
Monohybrid Test CrossUsed to determine the genotype
of an individual showing the dominant phenotype.
Cross the organism with the undetermined dominant genotype with an organism with the recessive genotype.
Example Test CrossSuppose in humans, tongue-
rolling is a dominant trait…we will represent it with a T.
We want to find the genotype of a man who can roll is tongue.◦ He could be TT…or Tt.
To test this, we would perform a cross between him and an individual who cannot roll their tongue. (tt)
Dihybrid CrossPredicts outcomes of a cross
looking at 2 traits at the same time.◦Example: Flower height and color,
Hair color and eye color.Punnett Square contains 16
boxes.
Law of Independent AssortmentGenes for different traits are
inherited independently of each other.
What does this mean?◦Things like eye color and hair color
are not “linked” or inherited together. Just because you inherit blue eyes from your mom does not mean you will also inherit her blonde hair.
Example
In pea plants, Round Seeds ( R) are dominant to Wrinkled Seeds (r) and Yellow Seeds (Y) are dominant to green seeds (y)
Practice ProblemIn humans, Brown hair (H) is
dominant to Blonde hair (h) and Brown eyes (B) are dominant to Blue eyes (b).
Create a punnett square crossing parents who are both heterozygous for both traits.
Incomplete DominanceWhen one allele is not
completely dominant over the other.
The heterozygous genotype shows a BLEND of the two homozygous phenotypes.
CodominanceBoth alleles contribute to the phenotype.
◦ In certain varieties of cows, the allele for a brown coat is codominant with the allele for a white coat.
◦ Heterozygous cows look like:
Multiple AllelesMore than 2 possible alleles exist
for a single gene.An individual cannot have more
than 2, but there are more than 2 possible alleles that can combine together.◦I.E. Not just a dominant and a
recessive.
ExampleIn blood typing, there are 3
alleles:◦A (IA), B (IB), and O (i).
The combination of 2 alleles that each individual person has, determines their blood type.
Blood Type Genotype Can Receive Blood From
A IA IA
IA i
A or O
B IB IB IB i
B or O
AB IA IB A, B, AB, or O
O ii O
aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC
AaBbCcAaBbCc
20/64
15/64
6/64
1/64
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