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C14 (Inheritance patterns are often more complex than predicted by simple…
C14
Inheritance patterns are often more complex than predicted by simple Mendelian genetics
Extending Mendelian Genetics for a Single Gene
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:
When alleles are not completely dominant or recessive
When a gene has more than two alleles
When a gene produces multiple phenotypes
Degrees of Dominance
incomplete dominance
the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
codominance
two dominant alleles affect the phenotype in separate, distinguishable ways
Complete dominance
when phenotypes of the heterozygote and dominant homozygote are identical
Tay-Sachs disease
fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain
At the organismal level, the allele is recessive
At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant
At the molecular level, the alleles are codominant
Dominant alleles are not necessarily more common in populations than recessive alleles
For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme that attaches A or B carbohydrates to red blood cells: IA, IB, and i
Most genes exist in populations in more than two allelic forms
Pleiotropy
Most genes have multiple phenotypic effects, a property called pleiotropy
For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
Epistasis
expression of a gene at one locus alters the phenotypic expression of a gene at a second locus
For example, in Labrador retrievers and many other mammals, coat color depends on two genes
One gene determines the pigment color (with alleles B for black and b for brown)
The other gene (with alleles E for color and e for no color) determines whether the pigment will be deposited in the hair
If heterozygous black labs (genotype BbEe) are mated, we might expect the dihybrid F2 ratio of 9:3:3:1
However, a Punnett square shows that the phenotypic ratio will be 9 black to 3 chocolate to 4 yellow labs
Epistatic interactions produce a variety of ratios, all of which are modified versions of 9:3:3:1
Many Human Traits follow Mendelian Patterns of Inheritance
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
Pedigrees can also be used to make predictions about future offspring
Recessively Inherited Disorders
Many genetic disorders are inherited in a recessive manner
The Behavior of Recessive Alleles
carriers
are heterozygous individuals who carry the recessive allele but are phenotypically normal
Most individuals with recessive disorders are born to carrier parents
If a recessive allele that causes a disease is rare, it is unlikely that two carriers will meet and mate
Consanguineous matings (i.e., between close relatives) increase the chance that both parents of a child carry the same rare allele
Recessively inherited disorders show up only in individuals homozygous for the allele
Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair
Cystic fibrosis
the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent
The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes, leading to a buildup of chloride ions outside the cell
Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
Sickle-Cell Disease
It is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
In homozygous individuals, all hemoglobin is abnormal (sickle-cell)
Symptoms include physical weakness, pain, organ damage, and even paralysis
A Genetic Disorder with Evolutionary Implications
Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common
Tests for Identifying Carriers
Fetal Testing
chorionic villus sampling
a sample of the placenta is removed and tested
amniocentesis
the liquid that bathes the fetus is removed and tested
Newborn Screening
One common test is for phenylketonuria (PKU), a recessively inherited disorder that occurs in one of every 10,000–15,000 births in the United States
Dominantly Inherited Disorders
Dominant alleles that cause a lethal disease are rare and arise by mutation
Achondroplasia is a form of dwarfism caused by a rare dominant allele
Huntington’s disease
is a degenerative disease of the nervous system
The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age
Once the deterioration of the nervous system begins, the condition is irreversible and fatal
Mendel used the scientific approach to identify two laws of inheritance
The Law of Segregation
When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple
When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
Mendel discovered a ratio of about three purple flowers to one white flower in the F2 generation
Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
Mendel called the purple flower color a dominant trait and the white flower color a recessive trait
The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation
Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits
What Mendel called a “heritable factor” is what we now call a gene
The Law of Independent Assortment
It states that each pair of alleles segregates independently of any other pair of alleles during gamete formation
This law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome
Genes located near each other on the same chromosome tend to be inherited together
Useful Genetic Vocabulary
alleles
alternative versions of a gene
a homozygote
An organism with two identical alleles for a character
It is said to be homozygous for the gene controlling that character
heterozygote
An organism with two different alleles for a gene
to be heterozygous for the gene controlling that character
not true breeding
we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup
testcross:
To determine the genotype we can carry out a testcross: breeding the mystery individual with a homozygous recessive individual
Probability laws govern Mendelian inheritance
The Multiplication and Addition Rules Applied to Monohybrid Crosses
addition rule
the probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities
The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous
multiplication rule
the probability that two or more independent events will occur together is the product of their individual probabilities
Probability in an F1 monohybrid cross can be determined using the multiplication rule
Segregation in a heterozygous plant is like flipping a coin: Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele
A multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously
In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied