Transcript chapter13_Sections 1

Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/chemistry/starr
Chapter 13
Observing Patterns in
Inherited Traits
(Sections 13.1 - 13.3)
Albia Dugger • Miami Dade College
13.1 Menacing Mucus
• Cystic fibrosis is the most common fatal genetic disorder in
the US – most CF patients live no more than 30 years
• The CFTR gene encodes a protein that moves chloride ions
out of epithelial cells, and binds disease-causing bacteria
• A deletion, ΔF508, disrupts membrane trafficking and causes
mucus to accumulate, making breathing difficult
CFTR Protein
• ATP-driven motors open a channel across the plasma
membrane; people with CF inherit 2 copies of ΔF508
13.2 Mendel, Pea Plants, and
Inheritance Patterns
• Gregor Mendel, an
Austrian monk, carefully
documented how
certain traits in pea
plants are inherited
Mendel’s Experimental Approach
• Mendel started with garden pea plants that “bred true” for a
particular trait, meaning that trait stayed the same in all
descendants, generation after generation
• When Mendel cross-fertilized pea plants with different traits,
the traits of the offspring appeared in predictable patterns
• Mendel concluded that hereditary information is passed from
one generation to the next in discrete units
Breeding Garden Peas
Breeding Garden Peas
B
A
carpel
anther
A Garden pea flower, cut in half. Male gametes form
in pollen grains produced by the anthers, and female
gametes form in carpels. Experimenters can control
the transfer of hereditary material from one flower to
another by snipping off a flower’s anthers (to prevent
the flower from self-fertilizing), and then brushing
pollen from another flower onto its carpel.
C
B In this example, pollen from a plant with purple
flowers is brushed onto the carpel of a whiteflowered plant.
C Later, seeds develop inside pods of the cross-fertilized
plant. An embryo in each seed develops
into a mature pea plant.
D Every plant that arises from this cross has
purple flowers. Predictable patterns such as
this offer evidence of how inheritance works.
D
Fig. 13.2, p. 190
Breeding Garden Peas
Fig. 13.2a.1, p. 190
Breeding Garden
Peas
A
carpel
anther
A Garden pea flower, cut in half. Male gametes form in pollen grains
produced by the anthers, and female gametes form in carpels.
Experimenters can control the transfer of hereditary material from
one flower to another by snipping off a flower’s anthers (to prevent
the flower from self-fertilizing), and then brushing pollen from
another flower onto its carpel.
Fig. 13.2a.2, p. 190
Breeding Garden Peas
B In this example, pollen
from a plant with purple
flowers is brushed onto the
carpel of a white-flowered
plant.
C Later, seeds develop inside
pods of the cross-fertilized
plant. An embryo in each seed
develops into a mature pea
plant.
D Every plant that arises from
this cross has purple flowers.
Predictable patterns such as
this offer evidence of how
inheritance works.
B
C
D
Fig. 13.2b-d, p. 190
ANIMATION: Crossing garden pea plants
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Inheritance in Modern Terms
• Today, we know that individuals of a species share certain
traits because their chromosomes carry the same genes
• The DNA sequence of each gene occurs at a specific location
(locus) on a particular chromosome
• locus
• Location of a gene on a chromosome
Loci of Some Human Genes
Loci of Some Human Genes
ribosomal RNA
skin pigmentation
fibrillin 1 (Marfan
syndrome)
(Tay–Sachs
disease)
15
17
(Canavan disease)
p53 tumor antigen
NF1
serotonin transporter
(neurofibromatosis)
BRCA1 (breast,
ovarian cancer)
Growth hormone
19
LDL receptor
(coronary artery
disease)
insulin receptor
brown hair color
green/blue eye color
(Warfarin resistance)
HCG, β chain
LH, β chain
dystrophin
(muscular
dystrophy)
(anhidrotic
ectodermal
dysplasia)
IL2RG (SCID-X1)
prion protein
(Creutzfeldt–
Jakob disease)
oxytocin
GHRH
(acromegaly)
20
X
XIST X chromosome
inactivation control
(hemophilia B)
(hemophilia A)
(red-deficient color
blind)
(green-deficient
color blind)
Fig. 13.3, p. 191
Modern Terms (cont.)
• Diploid cells have pairs of genes, on pairs of homologous
chromosomes
• The two genes of a pair may be identical (homozygous), or
they may be slightly different alleles (heterozygous)
• homozygous
• Having identical alleles of a gene
• heterozygous
• Having two different alleles of a gene
Genes on Chromosomes
• Any pair of genes on
homologous
chromosomes may vary
as alleles
• Different alleles may
result in different
versions of a trait
Genes on
Chromosomes
Genes occur in pairs on
homologous chromosomes.
The members of each pair of
genes may be identical, or
they may differ slightly, as
alleles.
Fig. 13.4, p. 191
ANIMATION: Genetic terms
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Modern Terms (cont.)
• The particular set of alleles that an individual carries is their
genotype, and their observable traits, such as flower color,
make up their phenotype
• genotype
• The particular set of alleles carried by an individual
• phenotype
• An individual’s observable traits
Modern Terms (cont.)
• Offspring resulting from a cross, or mating, between
homozygous individuals with different traits are hybrids
• Often, expression of one allele influences expression of the
other, and the outcome is visible in the hybrid phenotype
• hybrid
• Offspring of a cross between two individuals that breed
true for different forms of a trait; a heterozygous individual
Modern Terms (cont.)
• An allele is dominant when its effect masks that of a
recessive allele paired with it
• dominant
• An allele that masks the effect of a recessive allele paired
with it (indicated by italic capital letters: A)
• recessive
• An allele whose effect is masked by a dominant allele on
the homologous chromosome (italic lowercase: a)
Key Concepts
• Where Modern Genetics Started
• Gregor Mendel gathered evidence of the genetic basis of
inheritance
• His meticulous work gave him clues that heritable traits
are specified in units
• The units, which are distributed into gametes in
predictable patterns, were later identified as genes
ABC Video: Tiger Cubs Born in Zoo
13.3 Mendel’s Law of Segregation
• When Mendel crossed plants that bred true for purple flowers
with plants that bred true for white flowers, all of the offspring
had purple flowers
• A pea plant with two P alleles (PP) has purple flowers, and
one with two p alleles (pp) has white flowers
• The allele for purple (P) is dominant over the allele for white
(p), so the heterozygote (Pp) also has purple flowers
Law of Segregation (cont.)
• When homologous chromosomes separate during meiosis,
the gene pairs on those chromosomes separate also
• Each gamete that forms carries only one gene of a pair
• Homozygous dominant plants (PP) make (P) gametes
• Homozygous recessive plants (pp) make (p) gametes
• Heterozygous plants (Pp) make equal numbers of (P) and
(p) gametes
Law of Segregation (cont.)
• When homozygous dominant and homozygous recessive
plants are crossed (PP X pp), only one outcome is possible
• A gamete with a P allele meets a gamete with a p allele, and
all first generation (F1) offspring will be heterozygous
• Genotype = Pp
• Phenotype = purple
Gene Segregation
DNA replication
meiosis I
2
1
meiosis II
3
gametes (P)
gametes (p)
zygote (Pp)
Fig. 13.5.1-3, p. 192
Gene
Segregation
DNA replication
meiosis I
2
1
meiosis II
3
gametes (P)
gametes (p)
zygote (Pp)
male gametes
4
female gametes
Stepped Art
Fig. 13.5, p. 192
Punnett Squares
• Punnett square
• Diagram used to predict the genetic and phenotypic
outcome of a cross
Punnett Squares
female gametes
male gametes
4
Fig. 13.5.4, p. 192
Testcross
• Breeding experiments use testcrosses to determine whether
the tested individual is heterozygous or homozygous
• testcross
• Method of determining genotype in which an individual of
unknown genotype is crossed with one that is known to be
homozygous recessive
Monohybrid Cross
• Another breeding experiment, a monohybrid cross, checks
the dominance relationship for a single trait
• monohybrid cross
• Breeding experiment in which individuals identically
heterozygous for one gene are crossed
• Frequency of traits among offspring offers information
about the dominance relationship between the alleles
A Monohybrid Cross
• In a monohybrid cross between two Pp plants (Pp X Pp), the
two types of gametes can meet in four possible ways:
Sperm P meets egg P → zygote genotype PP
Sperm P meets egg p → zygote genotype Pp
Sperm p meets egg P → zygote genotype Pp
Sperm p meets egg p → zygote genotype pp
• The probability that second-generation (F2) offspring will have
purple flowers is 3 purple to 1 white, or a ratio of 3:1
A Monohybrid Cross
A Monohybrid Cross
parent plant
homozygous
for purple
flowers
parent plant
homozygous
for white
flowers
Pp
hybrid
two types of gametes
B A cross between the F1 offspring is a
monohybrid cross. The phenotype ratio in F2
offspring in this example is 3:1 (3 purple to 1
white).
Fig. 13.6, p. 193
A Monohybrid
Cross
Fig. 13.6a, p. 193
A Monohybrid
Cross
parent plant
homozygous
for purple
flowers
parent plant
homozygous
for white
flowers
pp
PP
Pp
hybrid
P
p
two types of gametes
Fig. 13.6a, p. 193
A Monohybrid
Cross
Fig. 13.6b, p. 193
A Monohybrid
Cross
B A cross between the F1 offspring is a monohybrid cross. The phenotype ratio in F2 offspring
in this example is 3:1 (3 purple to 1 white).
Fig. 13.6b, p. 193
A Monohybrid Cross
parent plant
homozygous
for purple
flowers
parent plant
homozygous
for white
flowers
PP
pp
Pp
hybrid
P
p
P
p
P
PP
Pp
p
Pp
pp
B A cross between the F1 offspring is a
monohybrid cross. The phenotype ratio in F2
offspring in this example is 3:1 (3 purple to 1
white).
two types of gametes
A All of the F1 offspring of a cross between two plants that breed true
for different forms of a trait are identically heterozygous. These
offspring make two types of gametes: P and p
Stepped Art
Fig. 13.6, p. 193
ANIMATIONs: Monohybrid cross
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Traits of Pea Plants
• Mendel found that all
these traits of pea
plants were inherited
in the same pattern in
F1 and F2 generations
Mendel’s Law of Segregation
• The 3:1 phenotype ratios in F2 offspring of monohybrid
crosses became the basis of Mendel’s law of segregation
• law of segregation
• The two members of each pair of genes on homologous
chromosomes end up in different gametes during meiosis
Key Concepts
• Insights From Monohybrid Crosses
• During meiosis, pairs of genes on homologous
chromosomes separate and end up in different gametes
• Inheritance patterns of alleles associated with different
forms of a trait can be used as evidence of such gene
segregation
Animation: Testcross