Chapter 29- Heredity

Differentiate between genotypes and phenotypes.

Genotype- a person's genetic makeup Phenotype- the expression of that genotype in the body

Define allele

one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome. Dominant- one allele masks or suppresses the expression of its partner, represented by a capital letter Recessive- a allele that is masked, represented by a lowercase letter. For recessive genes to be expressed, they must be present in double dose, in the homozygous condition.

Compare and contrast dominant-recessive inheritance with incomplete dominance and codominance.

*Dominant recessive inheritance - The interaction of dominant and recessive alleles. one allele variant completely masks the other. *Incomplete dominance - Is uncommon in humans. shows some of both traits *codominance - alleles are both expressed when present

Provide examples illustrating how gene expression may be modified by environmental factors.

-Environmental factors may influence genetic expression after birth, poor infant nutrition on brain growth, general body development, and height. -Sun tan -women with bulging muscles -birth defects due to mother taking medicines during pregnancy -genetic disorders -hormonal deficits during childhood can lead to abnormal skeletal growth

Describe briefly some approaches of gene therapy.

-Using a modified virus to deliver a functional cope of the affected gene into cells -Inject the "corrected" DNA directly into the patients' cell.

Describe the basis of extranuclear (mitochondria-based) genetic disorders.

37 genes are in the mitochondria -Mitochondrial genes are transmitted to the offspring almost exclusively by the mother -Researches suggest that Alzheimer's and Parkinson's may be a mitochondrial-based genetic disorder

Describe the mechanism of sex-linked inheritance.

A gene found only on the X chromosome is said to be X linked. Only 5% of the Y's DNA code for nonsexual characteristics corresponding to those on the X and only those regions can participate in crossovers with the X. Whenever a male inherits an X linked recessive allele its expression is never masked or dampened because there is no corresponding allele on his Y chromosome. Therefore it is always expressed, however females must have two X-linked recessive alleles to express such a disorder. Very few females exhibit X-linked conditions such as colorblindness. X-linked traits are typically passed from mother to son

List and explain several techniques used to determine or predict genetic diseases.

Genetic screening- can be used to detect certain genetic conditions before birth Two avenues for identifying carriers of detrimental genes: Pedigrees and blood tests -Pedigrees- traces a genetic trait through several generations and helps predict the future -Blood tests- used to screen for sickling gene and sophisticated blood chemistry tests and DNA probes can detect the presence of other unexpressed recessive genes Fetal testing: amniocentesis and chorionic villus sampling -Amniocentesis- sample from the amniotic sac, checked for enzymes and other chemicals that serve as markers for specific disease, then DNA markers are checked for genetic diseases and then karyotyped for chromosomal abnormalities -Chorionic villus sampling- suctions off bits of the chorionic villi from the placenta for examining. Allows for testing at 8 weeks, but waiting until 10th is recommended. Karyotyping can be done immediately, Maternal blood samples are used in clinic to test for fetal chromosomal abnormalities such as down syndrome.

Explain how polygene inheritance differs from that resulting from the action of a single pair of alleles

Polygene inheritance results in continuous or quantitative phenotypic variation between two extremes and explains many human characteristics. EX: skin color, height, metabolic rate, and intelligence

Describe how RNA-only genes and epigenetic marks affect gene expression.

RNA-only genes The second layer of control (development, regulation, and expression of genes) consists of the RNA-only genes, 80% of the genome. -"control freaks" act in similar ways to generally reduce the expression of particular genes- small nonconding RNA's, microRNA's, small interfering RNA's, and piwi-interacting RNA's. -small RNA's are important during development and in helping us fight off viral infections -protect us from transposons (elements of our own DNA that can act as aggressive jumping genes) -research on small RNA's to treat cancers, liver diseases, and macular degeneration. Epigenetic marks-the third level of gene controls -small chemicial tags to determine whether the DNA is available for transcription or silenced. -account for the inactivation (by methylation) of one the female's X chromosomes in the earyl embryo -Epigenetic marks or lack of them predispose a cell for transformation from normal to cancerous

Describe events that lead to genetic variability of gametes.

independent assortment of chromosomes, crossover of homologues and random fertilization of eggs by sperm all contribute to the genetic variability of gametes. *Chromosome segregation and independent assortment - chance determines how tetrads align on the meiosis I mataphase spindle. This leads to an amazing amount of variation in gametes. The two alleles that determine each trait are distributed to different gametes. *Crossover of homologues and gene recombination - the crossing over and exchange of chromosomal parts during meiosis I. *Random fertilization - Any one of these millions of sperm can fertilize any one of the woman's eggs. This equates to about 72 trillion possible different zygotes