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23-1
Chapter 23: Patterns of Gene Inheritance
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
23-2
Mendel’s Laws
Gregor Mendel was an Austrian monk who in 1860 developed certain laws of heredity after doing crosses between garden pea plants.
Gregor Mendel investigated genetics at the organismal level.
Examples of traits that can be observed at the organismal level include facial features (ex: big noses) that cause generations to resemble each other.
23-3
Gregor MendelMendel’s law of segregation:
1.) Each individual has two factors (called genes) for each trait (one from each parent).
2.) The genes segregate (separate) during gamete formation (i.e., meiosis).
3.) Each gamete contains only one gene for each trait (i.e., they are haploid).
4.) Fertilization gives the new individual two genes for each trait (one from each parent, restores diploid state).
23-4
Homologous Chromosomes
Genes
From Father From Mother
Diploid = Two copies of each type of chromosome
Loci = Physical position of a gene on a chromosome
Allele = Alternate forms of a gene:
Alleles have the same position (locus) on a pair of homologous chromosomes
23-5
Alleles code for the same trait.
Examples of alleles:-curly or straight (alleles), hair type (gene)-attached or unattached (alleles), ear lobe type (gene)
Chromosomes segregate during the formation of the gametes and each gamete has only one chromosome from each pair.
Fertilization gives each new individual two chromosomes again.
23-6
The Inheritance and Expression of a Single Trait
A capital letter indicates a dominant allele, which is expressed when present.
An example is W for widow’s peak.
A lowercase letter indicates a recessive allele, which is only expressed only in the absence of a dominant allele.
An example is w for a continuous or straight hairline.
23-8
Genotype and Phenotype
Genotype refers to the genes of an individual which can be represented by two letters or by a short descriptive phrase.
Homozygous means that both alleles are the same; for example, WW stands for homozygous dominant and ww stands for homozygous recessive.
23-9
Heterozygous means that the members of the allelic pair are different—for example, Ww.
Phenotype refers to the physical or observable characteristics of the individual – widow’s peak or straight hairline.
Both WW and Ww result in widow’s peak, two genotypes with the same phenotype.
23-10
Gamete FormationBecause homologous pairs separate during
meiosis, a gamete has only one allele from each pair of alleles (for a specific gene).
If the allelic pair is Ww, the resulting gametes would contain either a W or a w, but not both – (gametes are haploid).
Ww represents the genotype of an individual.Gametes that could be produced by this
individual are W or w.
23-12
One-Trait CrossesIn one-trait crosses, only one trait (such
as type of hairline) is being considered.
When performing crosses, the original parents are called the parental generation, or the P generation.
All of their children are the filial generation, or F generation.
Children are monohybrids when they are heterozygous for one pair of alleles.
23-14
If you know the genotype of the parents, it is possible to determine the gametes and use a Punnett square to determine the phenotypic ratio among the offspring.
W w
w
W WW Ww
Ww ww
23-15
Monohybrid crossGenotypes of parents are known
(both are heterozygous Ww)
Genotypic Ratio1 WW homozygous dominant2 Ww heterzygous3 ww homozygous recessive
Phenotypic Ratio3 widow’s peak1 straight hairline
23-16
The One-Trait TestcrossIt is not always possible to discern a
homozygous dominant from a heterozygous individual by inspection of phenotype (they have the same phenotype – both will have widow’s peak).
A testcross crosses the dominant phenotype with the recessive phenotype.
If a homozygous recessive phenotype is among the offspring, the parent must be heterozygous.
23-17
One-trait testcross
All offspring have dominantphenotype. Therefore the dominant parent (genotype we are tying to figure out) must be homozygous dominant.
?
23-18
?Offspring have dominant andrecessive phenotypes. Therefore the dominant parent (genotype we are tying to figure out) must be heterozygous dominant.
23-19
1.) Both a man and woman are heterozygous for tongue rolling. Tongue rolling is dominant over non-tongue rolling. What is the chance that their child will be a tongue roller?
Male Female
Tt Tt
T t T t
T
t
T t
TT
Tt
Tt
tt
GENOTYPE
GAMETES
MALE
FEMALE
Offspring Phenotypes3 Rollers1 Non-Roller
3 of 4 chances for rollerchild (75% chance).
23-20
2) Both you and your sibling are non-rollers and your parents are rollers. Tongue rolling is dominant over non-tongue rolling. What are the genotypes of your parents?
OFFSPRING tt
PARENTS T t
T
t
T t
TT
Tt
Tt
tt
23-21
The Inheritance of Many Traits
Independent AssortmentThe law of independent assortment states that
each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the gametes.
This law is dependent on the random arrangement of homologous pairs at metaphase.
23-23
Two-Trait CrossesIn two-trait crosses, genotypes of the
parents require four letters because there two alleles for each trait.
Gametes will contain one letter for each trait.
When a dihybrid (heterozygous for both traits) reproduces with another dihybrid the phenotypic results are 9 : 3 : 3 : 1.
23-24
Widow’s Peak is dominant over Straight Hairline W w
Short Fingers are dominant over Long Fingers S s
Phenotype Genotypes
Widow’s Peak / Short Fingers WWSS WWSs WwSS WsSs
Widow’s Peak / Long Fingers WWss Wwss
Straight HL / Short Fingers wwSS wwSs
Straight HL / Long Fingers wwss
23-25
Dihybrid cross (two traits)Widow’s PeakShort Fingers
Straight HairlineLong Fingers
Widow’s PeakShort Fingers
homozygous dominant homozygous recessive
23-27
The Two-Trait TestcrossA testcross is done to determine
genotype of individual that has dominant phenotypes (for both traits).
(Homozygous dominant or heterozygous for the two traits under consideration).
Cross heterozygote for both traits with homozygous recessive for both traits - results in 1 : 1 : 1 : 1 ratio.
23-29
Dominant Recessive
normal skin pigmentation albinismfreckles no frecklesbroad lips thin lipstongue roller non-tongue rollerPTC taster PTC non-tasterlarge eyes small eyesmigraine headaches no migraine headachesnormal foot arch flat feet
SELECTED TRAITS IN HUMAN HEREDITY
23-30
If a man that is homozygous recessive for eye size (i.e., has small eyes) and is homozygous dominant forfreckles (i.e., has freckles) has children with a womanthat is homozygous dominant for eye size (i.e., has large eyes) and is homozygous recessive for freckles (i.e., does not have freckles), what are the potential phenotypes and genotypes of their children?
Man llFF IF only for gametes
Woman LLff Lf only for gametes
ILFf All are heterozygous for both traits and show large eyes with freckles
GENOTYPE PHENOTYPEIF IF
Lf
Lf ILFfILFf
ILFf
23-31
If one of the children reproduces with another person that has the same genotype, what are the chances that they will have a child with large eyes and freckles?
LlFf X LlFf
LF
Lf
lF
lf
LF Lf lF lf
LLFF
LLFf
LlFF
LlFf
LLFf
LLff
LlFf
llffLlff llFf
llFF llFf
LlFf Llff
LlFF LlFf
large eyes/freckles
Large eyes/no freckles
Small eyes/freckles
Small eyes/no freckles
1/16
3/16
3/16
9/16 or 56 %
23-32
Genetic Disorders
Patterns of Inheritance
When studying human disorders, biologists often construct pedigree charts to show the pattern of inheritance of a characteristic within a family.
Genetic counselors construct pedigree charts to determine the mode (dominant or recessive) of inheritance of a condition.
23-33
Pedigree Analysis: determine how a genetic disorderis inherited, chances of offspring having a genetic disorder.
Unaffected male Unaffected female
Affected Male Affected female
“UNION”
OFFSPRING
23-34
Genetic Disorders: medical conditions caused by alleles inherited from parents, hereditary disorder.
Autosomal Genetic Disorders: genetic disorders caused byAlleles on autosomal chromosomes (non-sex Chromosomes – similar to somatic).
Autosomal Disorders can be:1) Autosomal Dominant2) Autosomal Recessive
Autosomal Dominant AA or Aa have disorder (phenotype)
aa
Aa Aa Aa
aa
aa
23-36
Autosomal recessive pedigree chart (Tay-sachs disease, Cystic fibrosis, PKU)
CARRIER – Has allele butis unaffected
* H
OW
DO
YO
U K
NO
W I
ND
IVID
UA
L I
S H
ET
ER
OZ
YG
OU
S?
*
23-37
Autosomal dominant pedigree chart (Neurofibromatosis, Huntington disease)*
HO
W D
O Y
OU
KN
OW
IN
DIV
IDU
AL
IS
HE
TE
RO
ZY
GO
US
? *
23-38
Polygenic Inheritance
Polygenic traits are governed by more than one gene pair (e.g., several pairs of genes may be involved in determining the phenotype).
23-39
Polygenic inheritance
Such traits produce a continuous variation representing a bell-shaped curve (Ex: height in humans).
23-40
Skin Color
The inheritance of skin color, determined by an unknown number of gene pairs, is a classic example of polygenic inheritance.
A range of phenotypes exist from very dark to very light.
The distribution of these phenotypes also follows a bell-shaped curve.
23-41
Polygenic Disorders
Many human traits, like allergies, schizophrenia, hypertension, diabetes, cancers, and cleft lip, appear to be due to the combined action of many genes plus environmental influences.
23-42
Multiple Allelic Traits
Inheritance by multiple alleles occurs when more than two alternative alleles exist for a particular gene locus.
A person’s blood type is an example of a trait determined by multiple alleles (A, B, and O).
***Each individual inherits only two alleles for these genes.
23-43
ABO Blood TypesA person can have an allele for an A
antigen (blood type A) or a B antigen (blood type B), both A and B antigens (blood type AB), or no antigen (blood type O) on the red blood cells.
Human blood types can be type A (IAIA or IA i), type B (IBIB or IBi), type AB (IAIB), or type 0 (ii).
Alleles: A, B, O
23-45
Incompletely Dominant TraitsCodominance means that both alleles are
equally expressed in a heterozygote. (Ex: sickle cell anemia)
Incomplete dominance is exhibited when the heterozygote doesn’t show the dominant trait but shows an intermediate phenotype, representing a blending of traits. (Ex: curly, wavy, or straight hair)
23-47
Sickle-Cell DiseaseSickle-cell disease is an example of a
human disorder controlled by incompletely dominant alleles.
Sickle cell disease involves irregular, sickle shaped red blood cells caused by abnormal hemoglobin.
HbA represents normal hemoglobin; and HbS represents the sickled condition.
23-48
HbAHbA individuals are normal; HbSHbS individuals have sickle-cell disease and HbAHbS individuals have the intermediate condition called sickle-cell trait.
Heterozygotes have an advantage in malaria-infested Africa because the pathogen for malaria cannot exist in their blood cells.
This evolutionary selection accounts for the prevalence of the allele among African Americans.