Download 4.3 Syllabus Points

Theoretical Genetics
 Genetic Terms:
 P = parental generation of a cross
 F1 = the first generation after the parental (the results
of the first cross)
 F2 = a cross between F1 individuals yields F2
 Genotype – the alleles of an organism as inherited
from the parents
 Phenotype – the characteristics of the organism due to
the interaction of the genotype and the environment
 Homozygous – having two identical alleles of a gene
 Heterozygous – having two different alleles of a gene
AA aa are homozygous
Aa are heterozygous
 Dominant allele – expressed phenotypically whether it
is homozygous or heterozygous
 Recessive allele – expressed phenotypically only in the
homozygous state
Capital letters are used to represent the dominant allele
Lower case for the recessive
Dominant allele always listed first
 Codominant alleles – pairs of alleles that both affect
the phenotype when present in the heterozygous state
 Locus – the particular position of a gene on
homologous chromosomes
 Carrier – an individual that has one copy of a recessive
gene that causes a genetic disease in individuals that
are homozygous for the gene
 Test cross – checking the genotype of an individual by
crossing it with a known homozygous recessive
B? X bb
The percentage phenotype give answer
 Determine genotypes and phenotypes using a punnett
square.
 The possible gametes of each individual are



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determined.
Each gamete will have one of the two alleles of the
parent.
Generate a punnett square and write the alleles for
parent 1 on the left side of the Punnett square.
Place the alleles for parent 2 above the square
Fill the squares for each parent
 The odds of 2 or more independent events occurring
together is the product of each independent event.
 In a cross between pea plants that are heterozygous for
flower color (Pp), what is the probability that the
offspring will be homozygous recessive?
 ½x½=¼
 Probability that events that can occur in 2 or more
different ways will occur, is the sum of the separate
problems.
 In a cross between pea plants that are heterozygous for
flower color (Pp), what is the probability of the
offspring being heterozygous?
 ¼+¼=½
 The odds of two different traits occurring together in
the same offspring is the product of each independent
trait occurring in the offspring.
 In which phase of meiosis does this event occur?
 What assumption did Mendel make for this law to
work?
 Some traits have more than two alleles
 Coat color in rabbits and cats
 Drosophila wings
 Human blood types
 Women – homozygous XX
 Men – heterozygous XY
 23rd pair chromosomes
 Dimorphic
 SRY gene on Y determines
Presence of male features
 Mothers can only contribute X chromosome
 Fathers can contribute either X or Y thus determine
gender of child
50% chance boy,
50% chance girl for
each fertilization event
 Nile crocodiles sand temperature around the eggs
during incubation affects gender - higher
temperatures favor males
 Turtles and lizards high temperatures favor females
 Many fish are hermaphrodites and under certain
environmental conditions will switch gender
 http://www.ornl.gov/hgmis/posters/chromosome/cho
oser.shtml
 X chromosome much larger than Y
 Many genes found on X that are not present on Y
 Genes on X with no counterpart on Y are called sex
linked genes
 Often affect one gender more than another
 Color Blindness and Hemophilia
 Caused by mutant gene recessive
 Encodes either clotting factor Vlll = h-A
 Or clotting factor lX = h-B
 Carried on X chromosome
 Heterozygous will clot normally as their good copy of
gene produces enough clotting factor
 If only mutant gene copy present will suffer from
excessive bleeding
 Red-green color blindness carried on X
 Blue color sensitivity is carried on chromosome 7 and
inherited as any autosomal gene
4 Sex-Linked Traits:
1. Normal Color Vision: A:
29, B: 45, C: --, D: 26
2. Red-Green ColorBlind: A: 70, B: --, C:
5, D: -3. Red Color-blind: A:
70, B: --, C: 5, D: 6
4. Green Color-Blind: A:
70, B: --, C: 5, D: 2
 Females can be homozygous or heterozygous for sex-
linked traits
 Females with one or two dominant alleles for a sex
linked trait will not exhibit the trait
 Females that are heterozygous for sex-linked traits are
called carriers
 Only females can pass on sex-linked traits to their sons
• The calico cat is the result of multiple alleles on the X
chromosome combines with X inactivation.
• One allele causes orange fur and the other causes black
fur.
• The male cat will be either orange or black.
• Only a heterozygous female will be calico with regions
that are orange and regions that are black based on the
X inactivation
 Predict genotype and phenotype using Punnett
Squares for all of the inheritance patterns mentioned
above
 Deduce the genotype and phenotype of individuals in
a pedigree chart