Transcript Mendelian Inheritance - Santa Susana High School

Mendelian Inheritance
• The concept of inheritance was first described
by the Monk Gregor Mendel as he
documented the inheritance patterns viewed
in flowering pea plants. He later assigned the
gene as the basic unit of inheritance.
Basics: The Vocabulary of Genetics
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character - a heritable feature (hair color, height, ...)
trait - variations of characters (brown vs blond)
true breeding plant - self pollination leads to a offspring of the same
variety (all white or all purple flowers)
hybridization - the cross pollination of two true breeding varieties of
plants
P - Generation = true breeding parents
F1Generation = hybrid offspring of the P cross
F2Generation = self-pollinated offspring of the F1
allele - alternate versions of a gene
dominant allele - codes for a trait that is always expressed (symbolized
by a capital letter)
recessive allele - has no noticeable contribution to an organism's
appearance if a dominant allele is also present(symbolized by a lower
cased letter of the dominant trait)
wildtype - the dominant trait expressed in the highest ratio in nature
genotype - organisms genetic makeup
phenotype - organisms outward appearance
homologous - having the same structure (two copies of the same
chromosome are homologous)
homozygous - having one allelic copy of agene, true breeding (PP or pp)
heterozygous - having 2 different alleles for a gene
Mendel's Model
• Alternate versions of genes account
for variations in inherited
characteristics.
• For each characteristic, an organism
inherits two alleles. One from each
parent.
• If 2 alleles inherited differ, the
dominant one is expressed
• Law of Segregation - Two alleles for a
heritable characteristic are
separated during gametogenesis and
end up indifferent gametes
• Law of independent assortment each pair of alleles segregates
independently of other pairs of
alleles during gamete formation
Mendel's Model
• 7 basic pea plant traits
– Was fortunate that traits
were found on different
chromosomes and that traits
were only the result of 2
different alleles
Punnett Square
• A punnett square can be used to predict the allelic
frequencies when true breeding parents are crossed.
– draw a square having twice the number of side boxes as number
of alleles.
– segregate the alleles on the side of the boxes (if more than one
trait you must segregate as groups)
– recombine the alleles in the boxes
– count the ratios
• Notice that the genotype ratio for the F1 cross is 1:2:2:1 & the
phenotype ratio is 3:1 with the dominant trait (purple) expressed in
the hybrids.
dihybrid crosses
• For dihybrid crosses the results are still predictable and are
as follows: R - round, r - wrinkled, Y - yellow, y - green
Multiplication- addition rule:
• Multiplication- addition rule:
• You can use the allelic frequencies to predict the outcome of a
cross.
• Given the Genotypes RrYy & rrYy, what is the frequency of offspring
exhibiting both recessive traits?
• rr = 1/2
• yy = 1/4
• 1/2 x 1/4 = 1/8
• Chance for both dominant?
• Rr = 1/2
• YY = 1/4
• Yy = 1/2
• 1/2 x (1/4 + 2/4) = 3/8