4 chapter 14~ mendel & the gene idea. mendelian genetics 4 character – heritable feature that...
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Leading to the Law of Segregation 4 Alternative versions of genes (alleles) account for variations in inherited characteristics 4 For each character, an organism inherits 2 alleles, one from each parent 4 If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance 4 Mendel’s Law of Segregation –The alleles for each character segregate (separate) during gamete production (meiosis).TRANSCRIPT
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Chapter 14~Mendel & The Gene
Idea
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Mendelian genetics Character – heritable feature that
varies among individuals: fur color, flower color, height
Trait – variant for a character: brown/white, tall/short
True-bred (self-pollination) – all offspring of same variety
Hybridization (cross-pollination) – crossing of 2 different true-breds
P generation (parents) F1 generation (first filial generation)
– Offspring of P generation F2 generation – offspring of F1
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Leading to the Law of Segregation Alternative versions of genes
(alleles) account for variations in inherited characteristics
For each character, an organism inherits 2 alleles, one from each parent
If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance
Mendel’s Law of Segregation– The alleles for each character
segregate (separate) during gamete production (meiosis).
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Genetic vocabulary……. Punnett square: predicts the
results of a genetic cross between individuals of known genotype
Homozygous: pair of identical alleles for a character
Heterozygous: two different alleles for a gene
Phenotype: an organism’s traits Genotype: an organism’s genetic
makeup Testcross: breeding of a recessive
homozygote X dominate phenotype (but unknown genotype)
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The Law of Independent Assortment Law of Segregation
involves 1 character. What about 2 (or more) characters?
Monohybrid cross (cross of 1 character) vs. dihybrid cross (cross of 2 characters)
Mendel’s Law of Independent Assortment– The two pairs of alleles
segregate independently of each other.
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Non-single gene genetics, I Spectrum of Dominance Complere dominance: dominant only
expressed Incomplete dominance: appearance between
the phenotypes of the 2 parents. Ex: snapdragons
Codominance: two alleles affect the phenotype in separate, distinguishable ways.– Ex: Tay-Sachs disease – enzyme to
metabolize lipids in brain doesn’t work: AA – all normal, Aa – ½ normal,
½ dysfunctional, aa – all dysfunctional
Multiple alleles: more than 2 possible alleles for a gene. – Ex: human blood types,
rabbit fur color
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Non-single gene genetics, II Pleiotropy: genes with multiple
phenotypic effect. – Ex: sickle-cell anemia, cystic
fibrosis Epistasis: a gene at one locus
(chromosomal location) affects the phenotypic expression of a gene at a second locus. – Ex: mice coat color – 1 gene
determines black/brown, 1 gene determines if there is pigment
Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character – Ex: human skin pigmentation
(3 genes) and height Nature vs Nurture
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Human disorders The family pedigree
– Circle – female; Square – male– Shaded – have trait; Not shaded – trait isn’t expressed
Recessive disorders:– Albinism– Cystic fibrosis – increase of concentration in extracellular
chloride causes mucus build up in lungs/digestive tract• Most common• Lethal • 1/2500 of European descent
– Tay-Sachs
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Human Disorders cont.– Sickle-cell – amino acid substitution in hemoglobin
protein• Shape causes clumping• 1/400 of African descent• Large # of heterozygous - Malaria
Dominant disorders– Achondroplasia - dwarfism– Huntington’s – degenerative disease of nervous system
• Appears btw 35-45• Fatal
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Human Disorders cont. Testing
– Amniocentesis – insert a needle into uterus
• Test on amniotic fluid
• 14th-16th week– Chorionic villus
sampling (CVS) – insert tube through cervix into uterus
• Take tissue from placenta
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Chromosomal Linkage Thomas Hunt Morgan – work
proved that genes are on specific chromosomes
Worked with Drosophilia melanogaster (fruit fly)
XX (female) vs. XY (male) Sex-linkage: genes located on a
sex chromosome Linked genes: genes located on
the same chromosome that tend to be inherited together
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Human sex-linkage SRY gene: gene on Y chromosome that triggers the development of testes Fathers= pass X-linked alleles to all daughters only (but not to sons) Mothers= pass X-linked alleles to both sons & daughters Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia
X-inactivation: 2nd X chromosome in females condenses into a Barr body (randomly) – genes are only expressed from one
Females may be a mosaic Barr body is reactivated in ovaries Ex. Tortoiseshell cats &
Calico cats (white determined by another gene)
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Chromosomal errors, I Nondisjunction: members of a
pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II
Aneuploidy: chromosome number is abnormal– Monosomy~ missing
chromosome–Turner syndrome
– Trisomy ~ extra chromosome - Down syndrome (pair 21), Kleinfelter syndrome
– Polyploidy~ extra sets of chromosomes – common in plants
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Chromosomal errors, II Alterations of chromosomal structure:
– Deletion: removal of a chromosomal segment
– Duplication: repeats a chromosomal segment
– Inversion: segment reversal in a chromosome
– Translocation: movement of a chromosomal segment to another