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Mendelian Genetics

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Renee Chalmers

on 29 November 2017

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Transcript of Mendelian Genetics

Genes and the Environment
Changes in the environment conditions can cause variations in the phenotypic outcome of one genotype
If A and B are mutually exclusive,
then (A or B) = P(A) + P(B)
ex. probability of having a boy OR a girl
1/2 + 1/2 =1

If A and B are independent,
then P(A and B) = P(A) X P (B)
ex. probability of having
1st child a boy AND 2nd child a girl
1/2 X 1/2 = 1/4
Bioethics Mini Discussion
Many ethical, social and medical issues surround human genetic disorders
Reproduction issues
Would you want to get a genetic screening test to find out if you have any genetic disorders?
Would you want your child to get a genetic screening test to find out if they have any genetic disorders?
Should people with genetic disorders be banned from reproducing?
used to trace the inheritance of a
particular trait through multiple generations
Mendelian Genetics
Civic issues such as ownership of genetic information, privacy, historical contexts, etc.
Who should be told if you test positive?
Only you? Your family? Your employer? Your insurance company?
Can/should your employer fire you if you test positive for a genetic disorder?
Autosomal Recessive Disorders
Disorders that are a recessive trait carried on autosomal chromosomes

Individuals that are heterozygous are carriers

ex. Tay Sachs Disease- inability to break down lipids on the brain
Leads to degeneration of motor and mental performance

ex. Sickle Cell Anemia- recessive allele codes for abnormal hemoglobin

Law of Dominance- when there are two different alleles for one trait, one is expressed and the other is not
Law of Segregation and Recombination- allele pairs for each trait separate during gamete formation and recombine during fertilization
Law of Independent Assortment- each allele pair separates independently of other gene pairs during gamete formation
Mendel's Laws
Rules of Probability
Test Cross
determines the genotype of an organism with a dominant phenotype
if all offspring are phenotypically dominant, unknown parent is homozygous
if 1/2 of offspring are phenotypically dominant and 1/2 are recessive, unknown parent is heterozygous
can be used to analyze passage of single gene traits from parent to offspring
Law of Dominance
Law of Segregation
Law of Independent Assortment
this law ONLY applies if genes for two traits are located on DIFFERENT pairs of homologous chromosomes
uses a dihybrid cross
punnett square- tool used to determine the probability of certain outcomes
Patterns of Inheritance
Incomplete Dominance
Multiple Alleles
Polygenic Inheritance
Incomplete dominance-blending inheritance where the
phenotype of the heterozygote is an intermediate
Codominance- full expression of both alleles in the heterozygote
Multiple Alleles-more than two allellic possibilities for a gene
ex. human ABO bloodtype

polygenic inheritance- two or more GENES contribute to a single phenotype; causes a continuum of phenotypes
ex. human height, skin color
at least three separate
genes code for
skin pigmentation
ex. petal color in hydrangeas
color is blue in acidic soil
and pink in basic soil
and purple in neutral soil
ex. fur color in different temperatures
Himalayan rabbits carry the C gene, required for development of pigment in fur, skin and eyes
The C gene is inactive above 35 degrees C and it maximally active form 15 C to 25 C
On extremities produces black fur because gene is active ex. ears, tip of nose, and feet
On central body part produced white fur because gene inactive so no pigment produced
dominant trait- widow's peak
recessive trait- attached earlobes
Autosomal Dominant
Lethal dominant alleles are more rare than lethal
recessive alleles

ex. Achondroplasia- short-limbed dwarfism
A zygote that inherits two dominant alleles does not survive

ex. Huntington’s Disease- neurodegenerative disorder
Caused by a late acting lethal dominant (~35-40)
Not eliminated from the population because it can
be passed on to the next generation
In a Mendelien cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive?
P (sperm containing p) = 1/2
P (egg containing p) = 1/2
P (egg AND sperm containing p)=
1/2 X 1/2 = 1/4
In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability of the offspring being a heterozygote?
P ( sperm containing P AND egg containing p)
= 1/2 x 1/2 = 1/4
P (sperm containing p AND egg containing P)
= 1/2 X 1/2 =1/4
P (offspring being heterozygous) 1/4 + 1/4 = 1/2
If the second generation couple decides to have another child, what is the probability the child will have a widow's peak?
What is the probability this child from the first question will have attached earlobes?
What is the probability the child will have a widow's peak AND attached earlobes?
F2 phenotypic ratio of monohybrid cross= 3:1
F2 phenotypic ratio of dihybrid cross = 9:3:3:1
F2 phenotypic ratio =1:2:1
F2 phenotypic ratio = 1:2:1
Be able to:
Connect the process of meiosis to the passage of traits from parent to offspring.
Apply mathematical routines to determine Mendelian patterns of inheritance.
Construct explanations of the influence of environmental factors on the phenotype of an organism.
Pose questions about ethical, social, or medical issues surrounding human genetic disorders.

Big Questions:
How does the chromosomal basis of inheritance provide an understanding of the transmission of genes from parent to offspring?
How do environmental factors influence the expression of the genotype in an organism?

cross unknown with a homozygous recessive

statistical test most frequently used to determine whether data obtained experimentally supports the hypothesis
can be used in genetics to determine if the predicted mode of inheritance is actually occurring
helps to determine whether genes are linked or not
Chi Square Test
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