Chapter 10 Genetics: Mendel and Beyond
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Transcript Chapter 10 Genetics: Mendel and Beyond
Chapter 10 Genetics:
Mendel and Beyond
Biology 101
Tri-County Technical College
Pendleton, SC
It’s a character trait
A character is a feature such as flower color
Best described as a gene
A trait is a particular form of a character, such as
white flowers
Best described as an allele
A heritable character trait is one that is passed
from parent to offspring
Before Mendel, blending was model of
choice…and it was logical to a point
Law of Segregation
Mendel and it’s all about flowers and sex
In sexually reproducing organisms, an individual
possesses 2 “factors” for each trait
One came from mommy and one from daddy
When that individual makes gametes, those
“factors” (alleles) will separate (segregate) from
each other and wind up in different gametes
**Gamete receives only ONE member of the pair
of alleles
Defining Definitions
Genotype is genetic constitution of the organism
It’s what in the genes
Phenotype is the physical expression of the
genotype
It’s what one sees—phenotype is the expression of
genotype
Dominant means always expressed (in the
phenotype)
Only one allele needed for its expression
DOES NOT MEAN “THE BEST TO HAVE”
Definitions, cont.
Recessive means “covered”, “hidden”, or
“masked” by presence of dominant allele
Recessive allele only expressed when present in
pairs
Homozygous means “same” or two copies
of same allele
Can be homozygous dominant or
homozygous recessive
TT or tt, respectively
Definitions, cont.
Heterozygous means “different” or “other”
Two different alleles of the “trait” (gene/allele) in
question (Tt)
Monohybrid means differing in only ONE trait
tall or short; yellow or green, etc.
Dihybrid means differing in TWO traits
tall and yellow; short and green
Monohybrid Cross
P = parental generation
F1 = first filial generation (first generation
of hybrid)
F2 = second filial generation (second
generation of a hybrid)
Punnett square is an application that allows
prediction of probability of genotypes/
phenotypes from a genetic cross
Test Cross
Test cross is dated concept that still serves useful
purpose
Estes and the field of corn
Defined as mating an individual of unknown
genotype with an individual homozygous recessive
for the trait
Figure the Punnett square for this one…you will see
it again!!
Law of Independent Assortment
In dihybrid crosses, parental traits appear in new
combinations of 2 of the phenotypic classes
such new combinations called recombinant phenotypes
Alleles of different genes assort independently of
one another during gamete formation
In other words, T does not always stay with Y and
vice versa
Law of Independent Assortment NOT as universal
as law of segregation
Independent Assortment, cont.
Applies to genes on separate chromosomes
but not necessarily to those that lie on same
chromosome
Simply states that chromosomes segregate
independently during formation of gametes
as do any two genes (alleles) on separate
chromosome pairs
Dihybrid Crosses
Review: dihybrid means two contrasting
traits
TTYY crosses with ttyy
Do the Punnett square and look for 9
genotypes and 4 phenotypes
Be sure and remember the phenotypic
ratio…you will see it again and again!!!
Incomplete Dominance
When heterozygous phenotype is intermediate,
gene is said to be governed by incomplete
dominance/lack of dominance
Looks like blending to be sure
Time for a Wally World story…Yeah!!!!
Red and white snapdragon cross produces all pink
offspring
All F1 to self-fertilize and predict F2
Black rooster and white hen = all gray chicks
Red bull and white cow = all roan calves
To dominate…or Not
Complete dominance results when single allele
produces enough protein to give maximum
phenotypic response
Incomplete dominance results when heterozygotes
show intermediate phenotype
Codominance results when two alleles at locus
produce two different phenotypes that both appear
in heterozygoes
Means both are fully expressed
Best example is ABO blood grouping
ABO Blood Grouping
Blood type determined by proteins on surface of
RBCs
Protein coded for by info in DNA
Alleles are IA and IB
There are six genotypes and four phenotypes
This is ABO only, we will discuss Rh factor
This one always gets me into trouble
Chalk talk time on ABO and Rh typing
Linkage
Association between markers (genes/alleles) on
same chromosome such that they do NOT show
random assortment and seldom recombine
Closer the markers, lower frequency of
recombination
Closer they are together, more likely they are to
STAY together
MAP units
Determining Sex
Sex (gender) determined by sex
chromosomes (23rd pair)
Females are XX and males are Xy
AFAWK, only thing on y chromosome is
SRY (male-determining) gene and gene for
hairy ears
Nature was not overly kind to males
Implications of XX and Xy
Sex-linked Disorders
Defined as being “tied” to X sex
chromosome
Sex-linked disorders affect males more
One mutant recessive allele leads to redgreen colorblindness, muscular dystrophy,
and hemophilia
Human mutations inherited as sex-linked
dominant must LESS common than
recessives
Sex-linked, cont.
Dominants appear in every generation and
individuals carrying harmful mutation (even
as heterozygotes) often fail to survive and/or
reproduce
Chalk talk time on hemophilia and/or color
blindness