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Chapter 14: Mendel and the Gene Idea (Concept 14.4: Many human traits…
Chapter 14: Mendel and the Gene Idea
Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance
Mendel discovered basic principles of heredity by breeding garden peas
Mendel's Experimental, Quantitative Approach
1857: Mendel began breeding peas in the abbey garden to study inheritance
Mendel chose to work w/ peas because there are many varieties
one variety has purple flowers while the other has white flowers
Character: a heritable feature that varies among individuals, such as flower color
each variant for a character such as purple or white color flowers is called a trait
Another advantage of using peas Is their short generation time and the large number of offspring from each mating
Mendel could control mating between plants
1) Removed stamens from purple flower
2) Transferred sperm bearing pollen from stamens of white flower to egg bearing carpal of purple flower
3) waited for pollinated carpel to mature into pod
4) planted seeds from pod
5) examined offspring: all purple flowers
Mendel made sure that he started his experiments w/ varieties that were true-breeding
these plants had produced only the same variety as the parent plant
plant w/ purple flowers is true breeding if seeds produced by self-pollination give rise to plants w/ purple flowers
Mendel cross-pollinated two contrasting, true breeding pea varieties
Hybridization
True breeding parents are referred to as the P generation
Offspring are the F1 generation
allowing the F1 hybrids to self pollinate produces the F2 generation
The Law of Segregation
When Mendel allowed the F1 replants to self-pollinate, the white flower trait appeared in the F2 generation
purple flowers were dominant, white flowers were recessive
Mendel observed the pattern in six other characters
crossed a true-breeding variety that produced smooth, round pea seeds w/ one that produced wrinkled seeds
Mendel's Model
Mendel developed a model to explain his 3:1 inheritance pattern
1) Alternative versions of genes account for variations in inherited characters
Alleles
we can relate this concept to chromosomes and DNA
2) For each character, an organism inherits two copies of a gene, one from each parent
somatic cells in a diploid organism has two sets of chromosomes
a genetic locus is represented twice in a diploid cell
4) law go segregation states that the two alleles for a heritable character segregate during gamete formation and end up in different gametes
3) If the two alleles at a locus differ, then one, the dominant allele, determines the appearance of the organism the other, the recessive allele, does not have not have a noticeable effect
Useful Genetic Vocabulary
Homozygote
an organism that has a pair of identical alleles for a gene encoding a character
also said to be homozygous for that gene
Heterozygote
an organism that has two different alleles for a gene
also said to be heterozygous for that gene
Phenotype: an organism's appearance
Genotype: genetic makeup
The Testcross
determine the genotype, we can cross an organism with an organism w/ a recessive genotype
Testcross: breeding an organism of unknown genotype w/ a recessive homozygote
The Law of Independent Assortment
Monohybrids: All F1 progeny produced in Mendel's crosses of true-breeding parents
this cross between heterozygotes is a monohybrid cross
Dihybrid Cross: cross between F1 hybrids
the key is to see what happens when F1 plants self-pollinate and produce F2 offspring
phenotypic ratio is 3:1
there can also be a phenotypic ratio of 9:3:3:1
states that two or more genes assort independently
only applies to genes located on different chromosomes or very far apart on the same chromosome
Concept 14.4: Many human traits follow Mendelian patterns of inheritance
Humans are not good subjects for genetic
Generation time is too long
Parents produce relatively few offspring
breeding experiments
Pedigree Analysis
A pedigree is a family tree that describes the interrelationships of parents and children across generations
inheritance patterns of particular traits can be traced and described using pedigrees
widows peak
if someone is able to taste PTC
We can use the multiplication and addition. rules to predict the probability of specific phenotypes
The Behavior of Recessive Alleles
Recessively inherited disorders show up only in individuals homozygous for the allele
inherit the recessive allele
Carrier: a heterozygous individual who has the recessive allele but is phenotypically normal
most individuals with recessive disorders are born to carrier parents
Albinism is a recessive condition
Consanguineous mating increase the chance that both parents of a child carry the same car allele
social and economic factors influence this as well
Cystic Fibrosis
most common lethal genetic disease
strikes one out of 2,500 people of European descent
alleles result in defective or absent chloride transport channels
mucus build up and abnormal absorption of nutrients in the small intestine
Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications
affects one out of 400 African Americanas
caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells
all hemoglobin is abnormal in homozygous individuals
Prescence of one sickle cell allele can affect the phenotype
at molecular level, the two alleles are codominant
heterozygotes are less susceptible to the malaria parasite
Heterozygotes are usually healthy
Dominantly Inherited Disorders
Dominant alleles that cause a lethal disease care rare and arise by mutation
Achondroplasia is a for dwarfism caused by a rare dominant allele
Heterozygous individuals have dwarf phenotype
Huntington's Disease
caused by lethal dominant allele that has no obvious phenotypic effects until the individual is 35 to 40 years of age
Fetal Testing
Aminocentesis: liquid that bathes the fetus is removed and tested
genetic disorders can be detected in the amniotic fluid
can be taken out at the 15th week of pregnancy
Chrionic Villus Sampling: a sample of the placenta is removed and tested
can be taken out as early as the 10th week of pregnancy
karyotyping can be carried out immediately
other techniques, such as ultrasound, allow the fetal health to be assessed visually in utero
Concept 14.2: Probability laws govern Mendelian inheritance
The Multiplication and Addition Rules Applied to Monohybrid Crosses
Multiplication Rule
multiply the probability of one event by the probability of the other event
This can also be applied to an F1 monohybrid cross
like flipping a coin
Addition Rule
probability that any one bot two or more mutually exclusive events will occur is calculated by adding their individual probabilities
Solving Complex Genetics Problems w/ the Rules of Probability
determines the probability of specific genotypes w/o having to make punnett squares
can use multiplication rule to determine the probability of each genotype in the F2 generation
Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics
Degrees of Dominance
Complete Dominance
phenotypes of the heterozygote and dominant homozygote are indistinguishable
The F! offspring in Mendel's pea crosses looked like the parents because one allele had complete dominance over the other
Incomplete Dominance
the phenotype of heterozygotyes is intermediate between the phenotypes of individuals homozygous for either allele
when red snapdragons are crossed w/ white ones, the hybrids have pink flowers
Codominance
two alleles each affect the phenotype in separate, distinguishable ways
human MN blood group is determined by codominant alleles
M and N molecules are on the red blood cells of individuals heterozygous for the M and N alleles
The Relationship Between Dominance and Phenotype
Round vs. Wrinkled pea seed shape
dominant allele codes for an enzyme that helps convert an unbranched form of starch to a branched form in the seed
recessive allele codes for a defective form of thus enzyme leading to unbranched starch where excess water enters the seed by osmosis
Tay-Sachs disease
inherited disorder in humans
brains cells of a child with the disease can't metabolize certain lipids because a crucial enzyme does not work properly
allele is recessive
at the molecular level, the normal allele and Tay-Sachs allele are codominant
Frequency of Dominant Alleles
Dominant Alleles aren't more common in populations than recessive alleles
one baby out of 400 in the US in born w/ extra fingers or toes
polydactyly
caused by the presence of a dominant allele
Multiple Alleles
most genes exist in more than two allelic forms
ABO blood groups are determined by the person's two alleles of the blood group gene
three alleles: IA, IB, and i
four types: A, B, AB, or O
these letters refer to two carbohydrates A and B
Pleiotropy
Moist genes have multiple phenotypic effects
pleiotropic alleles are responsible for multiple symptoms associated w/ hereditary diseases like sickle-cell disease and cystic fibrosis
Epistasis
phenotypic expression of a gene at one locus alters that of a gene at a second locus
labrador coat color depend on two genes
one gene determines the pigment color
other gene determines whether the pigment will be deposited in the hair
Polygenic Inheritance
Quantitative Characters: those that vary in the population along a continuum
indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
height is a good example: 180 genes affect height
skin color in humans
