Download exercise mendelian-genetics

Exercise 11
Mendelian Genetics
Ji-Ung Maeng
BIO 111
TAMU
Objectives
• Differentiate between homozygous and
heterozygous genotypes
• Explain the difference between genotype and
phenotype
• Describe the difference between monohybrid
and dihybrid cross
• Apply Mendel’s laws to several crosses
• Demonstrate how chromosome alignment in
metaphase I affects the alleles in haploid cells
• Differentiate between simple, incomplete and
co-dominance
Gregor Mendel
• 1865 Experiments in Plant-Hybridization
– Foundation for genetics and inheritance
– 7 pea plant traits through generations
• Inherited in a predictable pattern/ratio
• Some traits were hidden
– “Factors” – genes
• Affect inheritance
• Alleles occur in pairs on homologous
chromosomes form each parent
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Terminology
• Character (= a heritable feature), Trait (= each variant
for a character)
Ex) Eye color – blue, green, brown, etc
• Gene = Unit of heredity, maternal and parental
• Alleles = Form of gene, alternative version of gene
• Loci = The specific site on a chromosome where alleles
are located
• Genotype = All alleles of the organism
– Homozygous if both alleles are the same
– Heterozygous if alleles are different
• Phenotype = Expressed traits of the organism
– DOMINANT = Allele that masks counterpart’s expression
– Dominant does NOT mean better. It means just expression
– recessive = Alleles that are masked
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Terminology, cont’d
• Monohybrid Cross = Crossing one trait
• Dihybrid Cross = Crossing two traits
• Hybrid = Organism that is the result of
parents with different forms of the trait
• Unlinked Genes = Genes are on separate
chromosomes
• Linked Genes = Genes occurring on the
same chromosome
Dihybrid Cross
Mendel’s Laws of Inheritance
• First Law -- Segregation
– Each gamete has equal chance of possessing either
of the two alleles (form of a gene)
– Seperation of homologous chromosomes or Sister
chromatids to poles
• Second Law -- Independent Assortment
– Alleles sort independently and are randomly
distributed in gametes
– Aligning at metaphase plate
– Must be unlinked genes
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Segregation
Independent Assortment
Steps to Solve Monohybrid
Cross
• Id paternal and maternal genotype
• Set up squares
– For each column or row
• One parent’s gametes/alleles for each
column/row
• “Cross” or multiply the alleles
– One from mom with one from dad
• Look at the ratio of each geno- &
phenotype
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Monohybrid Example
• A rancher wants to cross a brown (BB) horse with
a white (ww) mare. Color is an unlinked gene
and brown is dominant.
• What are the genotypes and phenotypes for the
F1 (first offspring) generation?
B
Parents = BB x ww
B
w
Bw Bw
w
Bw Bw
F1 = 100% Bw
Brown?
White?
Monohybrid Example 2
• A rancher wants to cross a brown (Bb) horse with
a white (ww) mare. Color is an unlinked gene
and brown is dominant.
• What are the genotypes and phenotypes for the
F1 (first offspring) generation?
• How does Mendel’s First Law apply?
B
Parents = Bb x ww
b
w
Bw bw
w
Bw bw
F1 = 50% Bw; Brown
50% bw; White
Dihybrid Example
• A heterozygous red tall plant crosses with a
homozygous blue short plant. Red is dominant
and Tall is dominant. Both traits are unlinked.
• F1 genotypes and phenotypes?
• How does Mendel’s Second Law apply?
P = RrTt x rrtt
RT
rt
Rt
RrTt Rrtt
Red & Tall?
Red & Short?
Blue & Tall?
Blue & Short?
25%
25%
25%
25%
rT
rt
rrTt
rrtt
RRTT? 0%
RRTt? 0%
RRtt? 0%
RrTT? 0%
RrTt? 25%
Rrtt? 25%
rrTT? 0%
rrTt? 25%
rrtt?
25%
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Terminology
• Transposons = “jumping genes”
discovered by Barbara McClintock
– Move around gene loci changing the
phenotype
• Purple Indian Corn Kernels
• Incomplete Dominance = Blending of
traits for an intermediate trait
– Red + White = Pink
• Co-Dominance = Equal expression of two
alleles
– Blood types; AB, A, B, O
Incomplete
Dominance
Co-Dominance
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Lethal Inheritance
• If express a lethal trait, will usually abort
during development
– But if heterozygote then are a carrier of the
trait
– Can be passed recessively or dominantly
• Not always expressed in development
– Huntington’s Disease
• Passed on dominant gene
• Middle-age onset
• Neural degeneration
Post-lab Questions
• Define genotype & phenotype
• Define heterozygous & homozygous
• Compare incomplete dominance and codominance
• Explain the importance of Mendel’s Laws of
Inheritance and Meiosis
• Explain the role of heterozygosity in diseases such
as Huntington’s
• Set-up a cross between two heterozygous rabbits
for fur color
• Goggles next week!
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