Chapter 10 inheritance

characters 

heritable features that vary among individuals ~ flower color

traits 

each variant for a character

true-breeding

plants that produce offspring of the same variety when they self-pollinate

hybridization

mated two contrasting, true-breeding varieties 

alleles

alternative versions of a gene 

dominant allele, recessive allele

if the two alleles at a locus differ, then the —- determines the organism’s appearance, and the —- has no noticeable effect on appearance

the law of segregation 

the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

monohybrids 

individuals that are heterozygous for one character) 

Punnett square

a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

Testcross

an individual with dominant phenotype could be either homozygous dominant or heterozygous

dihybrids 

follows two characters at the same time, 

the multiplication rule 

the probability that two or more independent events will occur together is the product of their individual probabilities

the addition rule 

the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

complete dominance 

occurs when phenotypes of the heterozygote and dominant homozygote are identical

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

Tay-Sachs disease 

- results from mutations in gene encoding a subunit of a lysosomal enzyme

- causes accumulation of lipids in the brain and results in death

Pleiotropy

most genes have multiple phenotypic effects 

Cystic fibrosis

- most common lethal genetic disease in the United States, striking 1/2,500 people of European descent 

- results in defective/absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell 

Sickle-cell anemia

- caused by the substitution of a single amino acid in the hemoglobin protein (in red blood cells)

Epistasis

a gene at one locus alters the phenotypic expression of a gene at a second locus

Polygenic Inheritance

additive effect of two or more genes on a single phenotype

Quantitative characters 

are those that vary in the population along a continuum

multifactorial

traits that depend on multiple genes combined with environmental influences are called 

pedigree 

a family tree that describes the interrelationships of parents and children across generations

carriers 

heterozygous individuals who carry the recessive allele but are phenotypically normal → give rise to offspring with recessive disorders

Albinism 

a recessive condition characterized by lack of pigmentation in skin and hair

Dominantly Inherited Disorders

- some human disorders are caused by dominant alleles

- dominant alleles that cause a lethal disease are rare and arise by mutation

Achondroplasia

a form of dwarfism caused by a rare dominant allele (1/25,000 people)

Late-onset diseases

- the timing of onset of a disease significantly affects its inheritance

- a lethal dominant allele can escape elimination if it causes death at a relatively advanced age, after the individual has already passed on the lethal allele to his or her children

Huntington’s Disease: 

- a degenerative disease that causes deterioration of certain nerve cells in the brain 

- has no obvious phenotypic effects until the individual is about 35 to 40 years of age ~ uncontrolled movements, emotional disturbances and mental deterioration

Multifactorial disorders

- many diseases, such as heart disease, cancer, alcoholism, and mental illnesses, have both genetic and environmental components

- no matter what our genotype, our lifestyle has a tremendous effect on phenotype

Genetic Testing and Counseling

- Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

- Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals

Tests for Identifying Carriers

- For a growing number of diseases, tests are available that identify carriers and help define the odds of having an affected child more accurately

→ enable people to make more informed decisions about having children

amniocentesis

the liquid that bathes the fetus is removed and tested

chorionic villus sampling (CVS), 

a sample of the placenta is removed and tested

chromosome theory of inheritance

- Mendelian genes have specific loci (positions) on chromosomes

- chromosomes undergo segregation and independent assortment

wild type

or normal, phenotypes 

mutant phenotype

traits alternative to the wild type are called 

sex-linked genes

- genes that are located on either sex chromosome 

- there are few Y-linked genes (~78 genes, encodeing ~25 proteins)

hemizygous

a male needs only one copy of the allele 

X-linked disorders

color blindness, Duchenne muscular dystrophy, hemophilia

Barr body

the inactive X condenses into a 

genetic recombination 

the production of offspring with combinations of traits differing from either parent

parental types 

offspring with a phenotype matching one of the parental phenotypes

recombinant types 

offspring with nonparental phenotypes (new combinations of traits) 

genetic map 

an ordered list of the genetic loci along a particular chromosome 

linkage map

the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency 

nondisjunction

- pairs of homologous chromosomes or sister chromatids do not separate normally during meiosis

→ one gamete receives two of the same type of chromosome, and another gamete receives no copy

monosomic 

one copy of a particular chromosome

aneuploidy

offspring have abnormal number of a particular chromosome

trisomic 

three copies of a particular chromosome

Polyploidy

a condition in which an organism has more than two complete sets of chromosomes

deletion 

chromosomal fragment is lost

duplication 

deleted fragment may attaches to sister chromatid or nonsister chromatid of a homologous chromosome

inversion 

detached fragment reattaches in reverse orientation

translocation 

detached segment joins a nonhomologous chromosome

Down Syndrome (Trisomy 21)

an aneuploid condition that results from three copies of chromosome 21

Klinefelter syndrome 

XXY, ~1/1000 male births, do not exhibit any syndrome, often sterile

Turner syndrome (Monosomy X): 

X0, 1/2500 female births, sterile, only known viable monosomy in humans 

Genomic Imprinting

a variation in phenotype (for a few mammalian traits) that depends on which parent passed along the alleles for those traits