albia dugger miami dade college chapter 13 observing patterns in inherited traits
TRANSCRIPT
13.1 Menacing Mucus
• Cystic fibrosis (CF) is the most common fatal genetic disorder in the United States
13.2 Mendel, Pea Plants, and Inheritance Patterns
• Recurring inheritance patterns are observable evidence of how heredity works
• Before the discovery of genes, it was thought that inherited traits resulted from a blend of parental characters
Terms Used in Modern Genetics
• Genes are heritable units of information about traits
• Each gene has a specific locus on a chromosome
• Alleles are different molecular forms of a gene
Figure 13-3a p205
ribosomal RNA
skin pigmentation
15
(Tay–Sachs disease)
fibrillin 1 (Marfan syndrome)
Figure 13-3b p205
17
BRCA1 (breast, ovarian cancer)
NF1 (neurofibromatosis)
(Canavan disease)
serotonin transporter
Growth hormone
p53 tumor antigen
Figure 13-3c p205
19 LH, β chain
HCG, β chain
(Warfarin resistance)
green/blue eye color
brown hair color
insulin receptor
LDL receptor (coronary artery disease)
Figure 13-3e p205
(green-deficient color blind)X(red-deficient color blind)
(hemophilia A)
(hemophilia B)
XIST X chromosomeinactivation control
IL2RG (SCID-X1)
dystrophin (muscular dystrophy)
(anhidrotic ectodermal dysplasia)
Terms Used in Modern Genetics
• Phenotype: observable traits
• Any mutated gene is a new allele, whether or not it affects phenotype
Terms Used in Modern Genetics
• An allele is dominant if its effect masks the effect of a recessive allele paired with it• Capital letters (P) signify dominant alleles; lowercase
letters (p) signify recessive alleles• Homozygous dominant (PP)• Homozygous recessive (pp)• Heterozygous (Pp)
Genotypes Give Rise to Phenotypes
Pp(heterozygous atthe P gene locus)
genotype:
phenotype:
PP(homozygous fordominant allele P)
pp(homozygous forrecessive allele p)
13.3 Mendel’s Law of Segregation
• Pairs of genes on homologous chromosomes separate during meiosis, so they end up in different gametes
• Mendel showed that garden pea plants inherit two “units” of information for a trait, one from each parent
Gene Segregation
• Homologous chromosomes (and all the alleles they carry) segregate into separate gametes during meiosis
Calculating Probabilities
• Probability• A measure of the chance that a particular outcome will
occur
• Punnett square• A grid used to calculate the probability of genotypes and
phenotypes in offspring
Stepped Art
gametes (p)
meiosis II
gametes (P)
DNA replication
meiosis I
1 2
zygote (Pp)
3
female gametes
mal
e g
amet
es
4
Figure 13-5 p206
Monohybrid Crosses
• A monohybrid cross is a testcross that checks for a dominance relationship between two alleles at a single locus
• May be a cross between true breeding (homozygous) individuals (PP x pp), or between identical heterozygotes (Pp x Pp)
Generations in a Monohybrid Cross
• P stands for parents, F for filial (offspring)
• F1: First generation offspring of parents
• F2: Second generation offspring of parents
Mendel’s Monohybrid Crosses
• Mendel used monohybrid crosses to find dominance relationships among pea plant traits
• When he crossed plants that bred true for white flowers with plants that bred true for purple flowers, all F1 plants had purple flowers
• When he crossed two F1 plants, ¾ of the F2 plants had purple flowers, ¼ had white flowers
Testcrosses
• A testcross is a method of determining if an individual is heterozygous or homozygous dominant
• An individual with unknown genotype is crossed with one that is homozygous recessive (PP x pp) or (Pp x pp)
Mendel’s Dihybrid Cross
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
Mendel’s Law of Segregation
• Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses• Consistent with the probability of the pp genotype in the
offspring of a heterozygous cross (Pp x Pp)
• This is the basis of Mendel’s law of segregation• Diploid cells have pairs of genes on pairs of homologous
chromosomes • The two genes of each pair separate during meiosis, and
end up in different gametes
13.4 Mendel’s Law of Independent Assortment
• Mendel’s law of independent assortment• During meiosis, members of a pair of genes on
homologous chromosomes get distributed into gametes independently of other gene pairs
Dihybrid Crosses
• Dihybrid crosses test for dominance relationships between alleles at two loci
• Individuals that breed true for two different traits are crossed (PPTT x pptt)
• F2 phenotype ratio is 9:3:3:1 (four phenotypes)
• Individually, each dominant trait has an F2 ratio of 3:1 – inheritance of one trait does not affect inheritance of the other
The Contribution of Crossovers
• Independent assortment also occurs when the genes are on the same chromosome, but far enough apart that crossing over occurs between them very frequently
• Genes that have loci very close to one another on a chromosome tend to stay together during meiosis and not assort independently
Linkage Groups
• All genes on one chromosome are called a linkage group
• The farther apart two genes are on a chromosome, the more often crossing over occurs between them
• Linked genes are very close together; crossing over rarely occurs between them
• The probability that a crossover will separate alleles of two genes is proportional to the distance between those genes
13.5 Beyond Simple Dominance
• Mendel focused on traits based on clearly dominant and recessive alleles; however, the expression patterns of genes for some traits are not as straightforward
Codominance
• Codominance• Two nonidentical alleles of a gene are both fully expressed
in heterozygotes, so neither is dominant or recessive• May occur in multiple allele systems
• Multiple allele systems• Genes with three or more alleles in a population• Example: ABO blood types
Incomplete Dominance
• Incomplete dominance• One allele is not fully dominant over its partner• The heterozygote’s phenotype is somewhere between the
two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring
• Example: Snapdragon color• RR is red• Rr is pink• rr is white
Epistasis
• Epistasis• Two or more gene products influence a trait• Typically, one gene product suppresses the effect of
another
• Example: Coat color in dogs• Alleles B and b designate colors (black or brown)• Two recessive alleles ee suppress color
Pleiotropy
• A pleiotropic gene influences multiple traits
• Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder
13.6 Nature and Nurture
• Variations in traits aren’t always the result of differences in alleles – many traits are influenced by environmental factors
Environment and Gene Expression
• The environment affects the expression of many genes
• We can summarize this relationship as:
genotype + environment → phenotype
Environment and Epigenetics
• Environmentally driven changes in gene expression patterns can be permanent and heritable
• Example: Many environmental factors affect DNA methylation patterns, enhancing or suppressing gene expression
Figure 13-14a p212
c Maturecutting at lowelevation (30meters abovesea level)
a Maturecutting athigh elevation(3,060 metersabove sealevel)
b Maturecutting atmid-elevation(1,400 metersabove sealevel)
Figure 13-15b p213
B Electron micrographs comparing Daphnia body form that develops in the presence of few predators (left) with the form that develops in the presence of many predators (right). Note the difference in the length of the tail spine and the pointiness of the head. Chemicals emitted by thewater flea’s insect predators provoke the change.
Mood Disorders in Humans
• Environment is a factor in schizophrenia, bipolar disorder, depression, and other mood disorders
• Example: Stress-induced depression causes methylation-based silencing of a particular nerve growth factor – some antidepressants work by reversing this methylation
• Future treatments for many disorders may involve deliberate modification of epigenetic marks in one’s DNA
13.7 Complex Variations in Traits
• Individuals of most species vary in some of their shared traits
• Many traits (such as eye color) show a continuous range of variation
Continuous Variation
• Continuous variation• Traits with a range of small differences• The more factors that influence a trait, the more
continuous the distribution of phenotype
• Bell curve• When continuous phenotypes are divided into measurable
categories and plotted as a bar chart, they form a bell-shaped curve
Regarding the Unexpected Phenotype
• Phenotype results from complex interactions among gene products and the environment
• Enzymes and other gene products control steps of most metabolic pathways
• Mutations, interactions among genes, and environmental conditions may result in unpredictable traits
• Example: Camptodactyly can affect any fingers on either or both hands