Ch 29

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1

Diploid number (46) of chromosomes in all cells except gametes

a.23 pairs of homologous chromosomes
b.1 pair-sex chromosomes
i.Determine genetic sex (XX = female, XY = male)
c.22 pairs–autosomes - Guide expression of most other traits

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d. Alleles

i.Matched genes at same locus (location) on homologous chromosomes
1.Homozygous–alleles same for single trait
2.Heterozygous–alleles different for single trait
ii.Dominant-one allele masks (suppresses) its recessive partner
iii.Recessive expressed only if both alleles recessive

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e. Genotype

genetic makeup

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f. Phenotype

expression of genotype

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3. Sexual Sources of Genetic Variation

a.Each person unique due to three events
i.Independent assortment of chromosomes
1.During gametogenesis, maternal and paternal chromosomes randomly distributed to daughter cells
2.→ Gamete receives only one allele of the four alleles (2 maternal / 2 paternal) for each trait
ii.Crossover of homologues
1.Genes are normally passed to daughter cells as a unit, but sometimes they break → recombination
iii.Random fertilization of eggs by sperm

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c. Dominant-Recessive Inheritance

a.Few phenotypes traced to single gene
b.Most traits determined by multiple alleles or by interaction of several gene pairs
c.Dominant-Recessive Inheritance EXAMPLE
i.Dominant allele—capital letter; recessive allele—lowercase letter
ii.T = tongue roller and t = cannot roll tongue
iii.TT and tt are homozygous; Tt is heterozygous
iv.Offspring: 25% TT; 50% Tt; 25% tt
v.Dominant traits (for example, widow's peaks, freckles, dimples)
1.Dominant disorders uncommon; many lethal → death before reproductive age

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vi. Recessive Inheritance

1.Some recessive genes more desirable condition
2.E.g., normal vision—recessive; astigmatism—dominant
3.Most genetic disorders inherited as simple recessive traits
4.Albinism, cystic fibrosis, and Tay-Sachs disease

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d. Incomplete Dominance

i.Heterozygous individuals have intermediate phenotype
ii.Rare in humans; example—sickling gene
1.SS = normal Hb made
2.Ss = sickle-cell trait; both aberrant and normal Hb made; can suffer sickle-cell crisis under prolonged reduction in blood O2
3.ss = sickle-cell anemia; only aberrant Hb made; more susceptible to sickle-cell crisis

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e. Multiple-Allele Inheritance

i.Genes that exhibit more than two allele forms
ii.E.g., ABO blood grouping
iii.Three alleles (IA, IB, i) determine ABO blood type in humans
1.Each person has only two
2.IA and IB-codominant (both expressed if present)
3.i - recessive

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f. Sex-Linked Inheritance

i.Inherited traits determined by genes on sex chromosomes
1.X-linked genes - found only on X chromosome
2.X-linked recessive alleles always expressed in males
a.Passed from mothers to sons (e.g., hemophilia or red-green color blindness); never father to sons

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g. Polygene Inheritance

i. Traits reflecting actions of several gene pairs at different locations
ii. Results in continuous phenotypic variation between two extremes
iii. Examples: skin color, eye color, height, intelligence, metabolic rate

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5. Environmental Factors in Gene Expression

a.Maternal factors (e.g., drugs, pathogens) alter normal gene expression during embryonic development
b.Embryos developed phenotypes not directed by their genes
i.Phenocopies-environmentally produced phenotypes; mimic conditions caused by genetic mutations
c.Environmental factors influence gene expression after birth
i.Poor nutrition affects brain growth, body development, and height
ii.Childhood hormonal deficits can lead to abnormal skeletal growth and proportions

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7. Genetic Screening, Counseling, and Therapy

a.Newborn infants routinely screened for number of genetic disorders
i.Congenital hip dysplasia, imperforate anus, PKU, and other metabolic disorders
b.Two ways to identify gene carriers - Pedigrees and blood tests
i.Pedigrees - Trace genetic trait through several generations; helps predict future
ii.Blood tests and DNA probes can detect presence of unexpressed recessive genes
1.E.g., Tay-Sachs and cystic fibrosis genes
c.Fetal Testing
i.Used when known risk of genetic disorder; both invasive; both carry risk
1.Amniocentesis
2.Chorionic villus sampling (CVS)

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Genetic outcomes influenced by Small RNAs

1.Act directly on DNA, other RNAs, or proteins
2.Inactivate transposons, genes that tend to replicate themselves and disable or hyperactivate other genes
3.Control timing of apoptosis during development
4.Prevent translation of another gene
5.Linked to some cancers and mental disorders

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Genetic outcomes influenced by Epigenetic marks (chemical groups attached to DNA or histone proteins)

1.Determine whether DNA available for transcription (acetylation) or silenced (methylation)
2.Genomic imprinting
a.Tags certain genes with methyl group during gametogenesis as maternal or paternal; essential for normal development
b.Allows embryo to express only mother's or father's gene
c.For example, deletions in chromosome 15 result in disorders with different symptoms
i.Prader-Willi syndrome if inherited from father
ii.Angelman syndrome if inherited from mother

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Genetic outcomes influenced by Extranuclear (Mitochondrial) Inheritance

1.Some genes (37) in mitochondrial DNA (mtDNA)
2.Transmitted by mother in cytoplasm of egg
3.Errors in mtDNA linked to rare disorders
4.Muscle disorders and neurological problems, possibly Alzheimer's and Parkinson's diseases