chapter 13 observing patterns in inherited traits (sections 13.4 - 13.6)
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Chapter 13 Observing Patterns in Inherited Traits (Sections 13.4 - 13.6). 13.4 Mendel’s Theory of Independent Assortment. When homologous chromosomes separate during meiosis, either one of the pair can end up in a particular nucleus - PowerPoint PPT PresentationTRANSCRIPT
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Albia Dugger • Miami Dade College
Cecie StarrChristine EversLisa Starr
Chapter 13Observing Patterns in
Inherited Traits(Sections 13.4 - 13.6)
13.4 Mendel’s Theory of Independent Assortment
• When homologous chromosomes separate during meiosis, either one of the pair can end up in a particular nucleus
• Thus, gene pairs on one chromosome get sorted into gametes independently of gene pairs on other chromosomes
• Punnett squares can be used to predict inheritance patterns of two or more genes simultaneously
Dihybrid Cross
• In a dihybrid cross, individuals identically heterozygous for alleles of two genes (dihybrids) are crossed, and the traits of the offspring are observed
• dihybrid cross• Breeding experiment in which individuals identically
heterozygous for two genes are crossed• The frequency of traits among the offspring offers
information about the dominance relationships between the paired alleles
A Dihybrid Cross
• Start with one parent plant that breeds true for purple flowers and tall stems (PPTT ) and one that breeds true for white flowers and short stems (pptt)
• Each plant makes only one type of gamete (PT or pt)
• All F1 offspring will be dihybrids (PpTt) and have purple flowers and tall stems
A Dihybrid Cross (cont.)
• Then cross two F1 plants: a dihybrid cross (PpTt X PpTt)
• Four types of gametes can combine in sixteen possible ways
• In F2 plants, four phenotypes result in a ratio of 9:3:3:1
• 9 tall with purple flowers• 3 short with purple flowers• 3 tall with white flowers• 1 short with white flowers
Law of Independent Assortment
• Mendel discovered the 9:3:3:1 ratio in his dihybrid experiments – and noted that each trait still kept its individual 3:1 ratio
• Each trait (gene pair) sorted into gametes independently of other traits (gene pairs)
• law of independent assortment • During meiosis, members of a pair of genes on
homologous chromosomes get distributed into gametes independently of other gene pairs
Independent Assortment
Fig. 13.7, p. 194
meiosis II
meiosis I
A This example shows just two pairs of homologous chromosomes in the nucleus of a diploid (2n) reproductive cell. Maternal and paternal chromosomes, shown in pink and blue, have already been duplicated.
B Either chromosome of a pair may get attached to either spindle pole during meiosis I. With two pairs of homologous chromosomes, there are two different ways that the maternal and paternal chromosomes can get attached to opposite spindle poles.
C Two nuclei form with each scenario, so there are a total of four possible combinations of parental chromosomes in the nuclei that form after meiosis I.
D Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinationsof maternal and paternal chromosomes.
gamete genotype:
meiosis II
meiosis I
or
PtpTPTpt
Independent Assortment
B Either chromosome of a pair may get attached to either spindle pole during meiosis I. With two pairs of homologous chromosomes, there are two different ways that the maternal and paternal chromosomes can get attached to opposite spindle poles.
or
Fig. 13.7, p. 194
meiosis I
C Two nuclei form with each scenario, so there are a total of four possible combinations of parental chromosomes in the nuclei that form after meiosis I.
meiosis I
A This example shows just two pairs of homologous chromosomes in the nucleus of a diploid (2n) reproductive cell. Maternal and paternal chromosomes, shown in pink and blue, have already been duplicated.
meiosis II
D Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinationsof maternal and paternal chromosomes.
gamete genotype:
meiosis II
PtpTPTpt
Stepped Art
Independent Assortment
A Dihybrid Cross
Fig. 13.8, p. 195
parent plant homozygous
for purple flowers and long stems
PPTT pptt
dihybridPpTt
four types of gametes
parent plant homozygous
for white flowers and short stems
1
2
3
4
PPTT PPTt PpTT PpTt
PPTt PPtt PpTt Pptt
PpTT PpTt ppTT ppTt
PpTt Pptt ppTt pptt
pt
PT Pt pT pt
PP
Pt
pT
pt
A Dihybrid Cross
PT Pt pT pt
PT
Fig. 13.8.1-3, p. 195
A Dihybrid Cross
Fig. 13.8.4, p. 195
A Dihybrid Cross
Fig. 13.8, p. 195
parent plant homozygous
for purple flowers and long stemsPPTT pptt
dihybridPpTt
four types of gametes
parent plant homozygous
for white flowers and short stems
1
2
3
4
PPTT PPTt PpTT PpTt
PPTt PPtt PpTt Pptt
PpTT PpTt ppTT ppTt
PpTt Pptt ppTt ppttPT Pt pT pt
PT pt
PT Pt pT pt
PP
Pt
pT
pt
Stepped Art
A Dihybrid Cross
ANIMATION: Dihybrid cross
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The Contribution of Crossovers
• Genes that are far apart on a chromosome tend to assort into gametes independently because crossing over occurs between them very frequently
• Genes that are very close together on a chromosome are linked, they do not assort independently because crossing over rarely happens between them
• linkage group • All genes on a chromosome
Key Concepts
• Insights From Dihybrid Crosses• Pairs of genes on different chromosomes are typically
distributed into gametes independently of how other gene pairs are distributed
• Breeding experiments with alternative forms of two unrelated traits can be used as evidence of such independent assortment
ANIMATION: Crossover review
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13.5 Beyond Simple Dominance
• Mendel studied inheritance patterns that are examples of simple dominance, in which a dominant allele fully masks the expression of a recessive one
• Other patterns of inheritance are not so simple:• Codominance• Incomplete dominance• Epistasis• Pleiotropy
Codominance
• Codominant alleles are both expressed at the same time in heterozygotes, as in multiple allele systems such as the one underlying ABO blood typing
• codominant • Refers to two alleles that are both fully expressed in
heterozygous individuals
• multiple allele system • Gene for which three or more alleles persist in a
population
Codominance: ABO Blood Types
• Which two of the three alleles of the ABO gene you have determines your blood type
• The A and the B allele are codominant when paired• Genotype AB = blood type AB
• The O allele is recessive when paired with either A or B • Genotype AA or AO = blood type A• Genotype BB or BO= type B• Genotype OO = type O
Codominance: ABO Blood Types
Fig. 13.9, p. 196
Phenotypes (blood type):
Genotypes:
O
OO
BABA
AA
or
AO AB
BB
or
BO
Codominance: ABO Blood Types
ANIMATION: Codominance: ABO blood types
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Incomplete Dominance
• With incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes
• incomplete dominance • Condition in which one allele is not fully dominant over
another, so the heterozygous phenotype is between the two homozygous phenotypes
Incomplete Dominance: Snapdragons
• In snapdragons, one allele (R) encodes an enzyme that makes a red pigment, and allele (r) makes no pigment• RR = red; Rr = pink; rr = white
• A cross between red and white (RR X rr) yields pink (Rr)
• A cross between two pink (Rr X Rr) yields red, pink, and white in a 1:2:1 ratio
Incomplete Dominance: Snapdragons
Fig. 13.10, p. 196
homozygous (RR) x homozygous (rr) heterozygous (Rr)
A Cross a red-flowered with a white-flowered plant, and all of the offspring will be pink heterozygotes.
B If two of the pink heterozygotesare crossed, the phenotypesof the resulting offspring will occur in a 1:2:1 ratio.
Incomplete Dominance: Snapdragons
Fig. 13.10a, p. 196
Incomplete Dominance: Snapdragons
Fig. 13.10a, p. 196
homozygous (RR) x homozygous (rr) heterozygous (Rr)
A Cross a red-flowered with a white-flowered plant, and all of the offspring will be pink heterozygotes.
Incomplete Dominance: Snapdragons
Fig. 13.10b, p. 196
Incomplete Dominance: Snapdragons
Fig. 13.10b, p. 196
B If two of the pink heterozygotes are crossed, the phenotypes of the resulting offspring will occur in a 1:2:1 ratio.
Incomplete Dominance: Snapdragons
ANIMATION: Incomplete dominance
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• Some traits are affected by multiple gene products, an effect called polygenic inheritance or epistasis
• epistasis • Effect in which a trait is influenced by the products of
multiple genes
Epistasis
Epistasis: Labrador Retriever
• Labrador retriever coat color, can be black, brown, or yellow
Epistasis: Labrador Retriever
• A dominant allele (B) specifies black fur, and its recessive partner (b) specifies brown fur
• A dominant allele of a different gene (E ) causes color to be deposited in fur, and its recessive partner (e) reduces color• A dog with an E and a B allele has black fur• A dog with an E allele and homozygous for b is brown• A dog homozygous for the e allele has yellow fur
regardless of its B or b alleles
Epistasis: Labrador Retriever
Animation: Coat Color in Labrador Retrievers
Pleiotropy
• A pleiotropic gene influences multiple traits
• Mutations in pleiotropic genes are associated with complex genetic disorders such as sickle-cell anemia, cystic fibrosis, and Marfan syndrome
• pleiotropic • Refers to a gene whose product influences multiple traits
Pleiotropy: Marfan Syndrome
• In Marfan syndrome, mutations affect elasticity of tissues of the heart, skin, blood vessels, tendons, and other body parts
• Haris Charalambous died when his aorta burst – he was 21
ANIMATION: Pleiotropic effects of Marfan syndrome
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Animation: Comb Shape in Chickens
13.6 Complex Variation in Traits
• Phenotype often results from complex interactions among gene products and the environment
• Many traits show a continuous range of variation
Continuous Variation
• Some traits appear in two or three forms; others occur in a range of small differences (continuous variation)
• The more genes and environmental factors that influence a trait, the more continuous is its variation
• continuous variation • In a population, a range of small differences in a shared
trait
Continuous Variation (Cont.)
• If a graph line drawn around the top of the bars showing the distribution of values for a trait is bell-shaped (a bell curve) the trait varies continuously
• bell curve• Bell-shaped curve• Typically results from graphing frequency versus
distribution for a trait that varies continuously
Continuous Variation (Cont.)
• Human height and eye color are traits that vary continuously
Continuous Variation (Cont.)
Environmental Effects on Phenotype
• Environmental factors often affect gene expression, which in turn affects phenotype:• Seasonal change in animal fur colors• Spines grow in presence of predators • Different plant heights when grown at different altitudes
Environmental Effects on Phenotype
• In summer, the snowshoe hare’s fur is brown; in winter, white – offering seasonal camouflage from predators
Animation: Continuous Variation in Height
Environmental Effects on Phenotype
• Daphnia at right developed a longer tail spine and a pointy head in response to chemicals emitted by predatory insects
Environmental Effects on Phenotype
• Yarrow plant (Achillea millefolium) grew to different heights at three different elevations
Fig. 13.16, p. 199
C Plant grown at low elevation (30 meters above sea level)
A Plant grown at high elevation (3,060 meters above sea level)
B Plant grown at mid-elevation (1,400 meters above sea level)
Environmental Effects on Phenotype
Key Concepts
• Variations on Mendel’s Theme• Not all traits appear in Mendelian inheritance patterns• An allele may be partly dominant over a nonidentical
partner, or codominant with it• Multiple genes may influence a trait; some genes influence
many traits• The environment also influences gene expression
Menacing Mucus (revisited)
• The ΔF508 allele that causes cystic fibrosis in homozygotes may persist because it offers heterozygous individuals a survival advantage against certain deadly infectious diseases
• People who carry it may have a decreased susceptibility to typhoid fever and other bacterial diseases that begin in the intestinal tract