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Human Genetics
•
Phenotype: observed physical and
functional traits
•
Genotype: complete set of genes and alleles
•
Alleles: Different versions of homologous
genes ex. B and b
Human genetics
•
How are gametes made?
•
How does chromosome behavior affect
inheritance of traits?
•
Somatic cells are diploid.
•
Gametes are haploid, with only one set of
chromosomes
egg
polar
body
spermatogonium
primary
spermatocyte
secondary
spermatocyte
oogonium
primary
oocyte
secondary
oocyte
polar bodies
(will be degraded)
spermatids
meiosis ll
meiosis l
SPERMATOGENESIS OOGENESIS
a
b
1st law - segregation of alleles
•
Cells contain 2 copies (alleles) of each gene
•
Alleles separate during gamete formation
(meiosis)
•
gametes carry only one copy of each gene
Figure 19.2
Possible genotypes
and
their probabilities
Punnett squares
show parental
gametes
and the genotypes
of next generation
•
Homozygous: BB and bb
•
Heterozygous: Bb
Law of Independent Assortment
•
During gamete formation, genes for
different traits separate
independently into gametes
•
Why? random alignment of homologues at
Meiosis I
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 9.17
Chromosome behavior accounts for Mendel’s
principles
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
A B
a
b
Tetrad
Crossing over
A B
a
ba
B
A b
Gametes
•
Genes on the same chromosome tend to be
inherited together = linked genes
• Crossing over produces gametes with
recombinant chromosomes
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
•
an offspring’s
phenotype is
intermediate
between the
phenotypes of its
parents
Incomplete dominance
P GENERATION
F
1
GENERATION
F
2
GENERATION
Red
RR
Gametes R r
White
rr
Pink
Rr
R r
R R
r r
1
/
2
1
/
2
1
/
2
1
/
2
1
/
2
1
/
2
SpermEggs
Pink
Rr
Pink
rR
White
rr
Red
RR
Figure 9.12A
VARIATIONS ON MENDEL’S PRINCIPLES
[...]... three alleles for ABO blood type in humans IA, IB, i Copyright © 2003 Pearson Education, Inc publishing as Benjamin Cummings Polygenic traits - A single trait may be influenced by many genes • Quantitative traits Fraction of population skin color, height, eye color Copyright © 2003 Pearson Education, Inc publishing as Benjamin Cummings Skin pigmentation Genetic traits in humans can be tracked through family...• Incomplete dominance in human hypercholesterolemia GENOTYPES: HH Homozygous for ability to make LDL receptors Hh Heterozygous hh Homozygous for inability to make LDL receptors PHENOTYPES: LDL LDL receptor Cell Normal Mild disease... color, height, eye color Copyright © 2003 Pearson Education, Inc publishing as Benjamin Cummings Skin pigmentation Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits follows Mendel’s principles and the rules of probability Figure 9.8A Copyright © 2003 Pearson Education, Inc publishing as Benjamin Cummings • Family pedigrees are used to determine patterns... alleles – Examples: achondroplasia, Huntington’s disease Figure 9.9B Copyright © 2003 Pearson Education, Inc publishing as Benjamin Cummings Sex-linked disorders affect mostly males • Most sex-linked human disorders are due to recessive alleles – Ex: hemophilia, red-green color blindness These traits appear mostly in males Why? Figure 9.23A – If a male receives a single X-linked recessive allele from
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