Genetic Patterns of Inheritance

1
Genetic Patterns of Inheritance

• Wednesday, July 30

2
Figure 14.1 A genetic cross

• Mendels Breeding Experiments
• Hybridization (mating, cross) of contrasting,
  true- breeding organisms to follow a specific
  trait and study its mechanism of inheritance
• true-breeding self-pollinating plants produce
  offspring of the same variety
• character heritable feature (eye color, flower
  color, ect.)
• trait variant for a character (purple or white
  colored flowers)
• allele alternate versions of a gene

3
Principles of Heredity

• Law of segregation
• Two alleles for a character are packaged into
  separate gametes
• Law of independent assortment
• Each pair of alleles segregates into gametes
  independently

4
Figure 14.2 Mendel tracked heritable characters
for three generations
F1 generation yields all purple flowers. The
white trait is not lost because it reappears in
the F2 generation. The heritable factor must be
a discrete unit that retains a separate identity
in offspring (genes) Purple dominant White
recessive
Self-pollinate
5
Table 14.1 The Results of Mendels F1 Crosses
for Seven Characters in Pea Plants
6
Principles of Inheritance

• Alternative versions of genes (alleles) account
  for variations in inherited characters (purple
  allele, white allele)
• For each character, an organism inherits two
  alleles, one from each parent
• If the two alleles differ, the dominant allele is
  fully expressed in the organisms appearance the
  recessive allele has no effect
• The two alleles for each character segregate
  during gamete production

7
Figure 14.3 Alleles, alternative versions of a
gene
8
Principles of Inheritance

• Alternative versions of genes (alleles) account
  for variations in inherited characters (purple
  allele, white allele)
• For each character, an organism inherits two
  alleles, one from each parent
• Diploid organisms have 2 sets of homologous
  chromosomes (one from each parent), so each gene
  locus is represented twice
• If the two alleles differ, the dominant allele is
  fully expressed in the organisms appearance the
  recessive allele has no effect
• The two alleles for each character segregate
  during gamete production
• meiosis produces gametes with only one of the two
  alleles

9
Figure 14.4 Mendels law of segregation
The parents are true-breeding plants, meaning
they have identical alleles (PP or pp) for this
trait Gametes all have one allele that are the
same as the parental allele The first
generation (F1) is a hybrid of the parental
alleles (Pp). Since purple (P) is dominant, all
the progeny are purple. When the F1 hybrids
produce gametes, the alleles segregate, giving
half the P allele and half the p allele
Upper case letters (P) designates a dominant
allele (purple) Lower case letters (p) designates
a recessive allele (white)
10
Figure 14.4 Mendels law of segregation
The F1 generation is self-fertilized to produce
the second (F2) generation. This cross can be
displayed by a Punnett square, which shows all
the possible combinations of alleles for the
offspring. Each box represents a 25 probable
product. Genotype ¼ PP (homozygous
dominant) ½ Pp (heterozygous) ¼ pp
(homozygous recessive) Phenotype ¾ purple ¼
white
11
Figure 14.5 Genotype versus phenotype
12
Figure 14.6 A testcross
13
Principles of Heredity

• Law of segregation
• Two alleles for a character are packaged into
  separate gametes
• Law of independent assortment
• Each pair of alleles segregates into gametes
  independently

14
Dihybrid Crosses

• Following a single characteristic (color)
• F1 produced in these crosses are monohybrids
• Following two characteristics (color and shape)
• Seed color yellow (dominant, Y) or green
  (recessive, y)
• Seed shape round (dominant, R) or wrinkled
  (recessive, r)
• YYRR x yyrr ? YyRr (dihybrids)
• Are two characteristics (color and shape)
  inherited together or independently?

15
Figure 14.7 Testing two hypotheses for
segregation in a dihybrid cross
Four classes of gametes will be produced
Only two classes of gametes will be produced,
those of the parental gametes
9331
31
16
Figure 14.8 Segregation of alleles and
fertilization as chance events
Rules of Probability application to genetic
inheritance Rule of multiplication - Chance that
two independent events occur together chance
that one event occurs x chance that the other
event occurs (chance of flipping 2 coins tails up
½ x ½ ¼) Rule of Addition If an event can
occur by two or more ways (heterozygote can occur
by either Pp or pP) then the probability that the
event occurs is the sum of the probabilities of
each path (½ x ½ ¼)
17
Complex Patterns of Inheritance

• Dominance
• Multiple alleles
• Pleiotropy
• Epistasis
• Polygenic inheritance
• Environmental impact of phenotype

18
Dominance

• Complete dominance
• Incomplete dominance
• One allele is not completely dominant over the
  other heterozygotes are intermediate
• Codominance
• Both alleles are dominant - heterozygotes retain
  both characters (blood groups M, N, MN)
• Both dominant and recessive alleles are expressed
  in heterozygotes
• Dominance reflects the mechanism by which a gene
  is responsible for phenotype
• Eg one normal allele and one defective if half
  the amount of normal protein (or enzyme) is
  sufficient, then the normal allele is dominant
  (Tay-sachs disease). If it is not enough, then
  the defective allele is dominant (round/wrinkled
  seeds).

½
19
Figure 14.9 Incomplete dominance in snapdragon
color
20
Dominance

• Complete dominance
• Incomplete dominance
• One allele is not completely dominant over the
  other heterozygotes are intermediate
• Codominance
• Both alleles are dominant - heterozygotes retain
  both characters (blood groups M, N, MN)
• Both dominant and recessive alleles are expressed
  in heterozygotes
• Dominance reflects the mechanism by which a gene
  is responsible for phenotype
• Eg one normal allele and one defective if half
  the amount of normal protein (or enzyme) is
  sufficient, then the normal allele is dominant
  (Tay-sachs disease). If it is not enough, then
  the defective allele is dominant (round/wrinkled
  seeds).

½
21
Complex Patterns of Inheritance

• Dominance
• Multiple alleles for a single gene
• Pleiotropy
• Epistasis
• Polygenic inheritance
• Environmental impact of phenotype

22
Figure 14.10 Multiple alleles for the ABO blood
groups
Four blood groups result from the presence or
absence of 2 different carbohydrate groups (A and
B) found on the surface of RBCs. The genotype
results from the combination of 3 different
alleles IA A group IB B group i no
group There are 6 different possible
genotypes A and B are codominant and i is
recessive
23
Figure 14.10 Multiple alleles for the ABO blood
groups
6 genotypes determined by 3 alleles
Carbohydrate on RBC
Universal acceptor
Universal donor
24
Complex Patterns of Inheritance

• Dominance
• Multiple alleles for a single gene
• Pleiotropy
• ability of a gene to have multiple phenotypic
  effects
• Epistasis
• Polygenic inheritance
• Environmental impact of phenotype

25
Complex Patterns of Inheritance

• Dominance
• Multiple alleles for a single gene
• Pleiotropy
• ability of a gene to have multiple phenotypic
  effects
• Epistasis
• A gene at one locus alters the phenotypic
  expression of a gene at a second locus
• Polygenic inheritance
• Environmental impact of phenotype

26
Figure 14.11 An example of epistasis
Mouse coat color gene Black (dominant) B Brown
(recessive) b Color deposition gene Color
(dominant) C White albino (recessive)
c Because the C/c gene is dominant to the B/b
gene we say that C/c is epistatic to B/b.
27
Complex Patterns of Inheritance

• Dominance
• Multiple alleles for a single gene
• Pleiotropy
• single gene has multiple phenotypic effects
• Epistasis
• A gene at one locus alters the phenotypic
  expression of a gene at a second locus
• Polygenic inheritance
• An additive effect of two or more genes on a
  single phenotypic character (opposite of
  pleiotropy)
• Quantitative characters vary along a continuum
  (height)
• Environmental impact of phenotype

28
Figure 14.12 A simplified model for polygenic
inheritance of skin color
Skin color is controlled by 3 genes A/a, B/b,
C/c The dark skin alleles are incompletely
dominant to the light skin alleles. Each dark
skin allele contributes one unit of darkness to
the phenotype. AABBCC very dark aabbcc very
light AaBbCc and AABbcc intermediate
29
Complex Patterns of Inheritance

• Dominance
• Multiple alleles for a single gene
• Pleiotropy
• single gene has multiple phenotypic effects
• Epistasis
• A gene at one locus alters the phenotypic
  expression of a gene at a second locus
• Polygenic inheritance
• An additive effect of two or more genes on a
  single phenotypic character (opposite of
  pleiotropy)
• Quantitative characters vary along a continuum
  (height)
• Environmental impact on phenotype
• Multifactorial characters exercise on weight,
  nutrition on height, sun exposure on skin color

30
Figure 14.14 Pedigree analysis
Human Genetics
To study inheritance in humans, geneticists must
rely on the results of previous matings and
family trees. A family tree following a
particular trait is called a pedigree.
Dominant trait widows peak is caused by a
dominant allele (W)
Recessive trait attached earlobes are caused by
a recessive allele (f)
31
Inherited Human Disorders

• Recessively inherited disorders
• Disease is caused by the gene coding for a mutant
  protein or no protein
• Heterozygotes are normal because one normal gene
  is sufficient
• They are carriers of the gene since they may
  transmit the allele to their offspring
• If two carriers mate, the offspring have a 25
  chance of having the disorder
• Disorders are more prevalent is certain
  populations
• cystic fibrosis white Europeans (1/2500)
• Tay-sachs disease jewish central Europeans
• sickle-cell anemia African Americans (1/400)

32
Figure 14.15 Pleiotropic effects of the
sickle-cell allele in a homozygote
33
Inherited Human Disorders

• Dominantly inherited disorders
• Most disease are not lethal achondroplasia
  (dwarfism) occurs in 1/10,000 people
• 99.99 of the population are homozygous recessive
• Lethal disease can occur if it is late-acting
• Huntingtons disease no obvious phenotypic
  effect until 35-45 years old, when the allele may
  have already been passed on to offspring
• Multifactorial disorders
• Heart disease, cancer, diabetes, alcoholism
• Hereditary component is polygenic
• Environmental component affects risk

34
Genetic Testing and Counseling

• Using pedigrees and the rules of heredity we have
  learned, it is possible to predict the
  probability of offspring phenotypes
• Genetic risk for disorders can be assess by
  testing parents for heterozygousity
• Many diseases now have test for detecting
  carriers
• Fetal testing can determine whether or not a
  fetus has a disorder
• Ultrasound
• Amniocentesis
• CVS chorionic villus sampling

35
Figure 14.17 Testing a fetus for genetic
disorders
Chromosomal number and appearance
Chemicals can reveal presence of disease
36
Connecting genes to chromosomes

• Genes have specific loci on chromosomes
• It is the chromosomes that undergo segregation
  and independent assortment
• Some genes sort according to sex
• Sex-linked genes genes located on a sex
  chromosome
• Some genes are not assorted independently linked
  genes
• Each chromosome has 100s-1000s of genes
• Genes located on the same chromosome tend to be
  inherited together because the chromosome is
  passed along as a unit

37
Figure 15.3 Sex-linked inheritance
W dominant red eyes W recessive white
eyes Gene located on the X chromosome
38
Connecting genes to chromosomes

• Genes have specific loci on chromosomes
• It is the chromosomes that undergo segregation
  and independent assortment
• Some genes sort according to sex
• Sex-linked genes genes located on a sex
  chromosome
• Some genes are not assorted independently linked
  genes
• Each chromosome has 100s-1000s of genes
• Genes located on the same chromosome tend to be
  inherited together because the chromosome is
  passed along as a unit

39
Linked Genes are Inherited Together
40
Figure 15.4 Evidence for linked genes in
Drosophila
41
Figure 15.5a Recombination due to crossing over
Genetic recombination of linked genes crossing
over

• Crossing over exchange of segments between
  homologous chromosomes occasionally breaks
  linkage
• When homologous chromosomes are paired at meiosis
  I, nonsister chromatids break and switch
  fragments at corresponding points

Parental types
42
Figure 15.5b Recombination due to crossing over
43
Figure 15.6 Using recombination frequencies to
construct a genetic map

• The chance of crossing over is equal at all
  points on a chromosome
• The greater the distance between two genes, the
  more points where crossing over can occur
• The farther apart two genes are on a chromosome,
  the higher the recombination frequency

44
Figure 15.10 X inactivation and the
tortoiseshell cat

• Females have two X chromosomes
• One X chromosome in each cell becomes inactivated
• Males and females have the same effective dose
  (one copy) of genes with loci on the X chromosome
• Inactivation occurs randomly and independently in
  each cell
• Mosaic of two cell types

45
Errors in Chromosomal Inheritance

• Alteration in chromosome number
• Nondisjunction
• A pair of homologous chromosomes do not separate
  during meiosis I
• Sister chromatids do not separate during meiosis
  II
• Aneuploidy abnormal chromosome number
• Trisomic three copies of a chromosome
• Monosomic one copy of a chromosome
• Polyploidy more than 2 complete chromosomal
  sets
• Alteration in chromosome structure
• Breakage of a chromosome deletion, duplication,
  inversion, or translocation

46
Figure 15.13 Alterations of chromosome structure