Download 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