Transcript Chapter 7: Extending Mendelian Genetics

Chapter 7: Extending
Mendelian Genetics
Section 1: Chromosomes &
Phenotype
Autosomal Genes &
Phenotype
• Phenotype is determined by 2
copies of each autosomal gene
• Mendel studied autosomal
characteristics
• Most traits in humans are
determined by autosomal genes
Autosomal Disorders
• Many genetic disorders are caused by
recessive alleles on autosomes
– NN: healthy
– Nn: healthy; carrier for the disorder
– nn: has disorder
• Less genetic disorders are caused by
dominant alleles on autosomes
– NN and Nn: has disorder
– nn: healthy
Sex-Linked Traits
• Genes located on the sex chromosomes (X
or Y) are called sex-linked genes
• Genes on the Y chromosome are
responsible for male characteristics only
• X chromosome has many more genes that
affect many traits
• In males, all sex-linked traits are expressed
– they only have one copy of each chromosome,
so all alleles are expressed even if they’re
recessive
X Chromosome Inactivation
• Since females have 2 X
chromosomes, one of them is
randomly turned off in each cell
• Because it is random, a female will
have some cells with one X turned
off, and others with the other X
turned off
Section 2: Complex Patterns
of Heredity
Complex Patterns
• Incomplete dominance - heterozygous
individual shows a blending of the two
homozygous phenotypes
• Ex. Four o’clock plants
–RR = red flowers
–rr = white flowers
–Rr = pink flowers
• Codominance - both alleles are
dominant, so both are fully and
separately expressed
• Ex. Human ABO blood type - has
multiple alleles (IA, IB, & i)
– IAIA or IAi = type A blood
– IBIB or IBi = type B blood
– IAIB = type AB blood
– ii = type O blood
• Polygenic traits - traits produced by
two or more genes, leading to a
range of phenotypes
• Ex. human skin tone, human eye
color
• In a polygenic trait, one gene may
be epistatic, meaning it can interfere
with the expression of the other
genes
– Ex. albinism in mammals
Genes & the Environment
• Phenotype is usually a mixture of
genes & the environment
• Ex. genes determine your
underlying skin tone & hair color,
but sunlight can cause your skin to
become darker and hair to become
lighter
Section 4: Human Genetics &
Pedigrees
Human Genetics
• Humans follow the same basic
patterns of heredity
• The inheritance of many human traits
can be very complex
– Polygenic traits, the environment, etc…
• Some human traits are single-gene
traits that follow simple dominant and
recessive patterns like Mendel’s peas
– Ex. widow’s peak, Huntington’s disease
X and Y Chromosomes
• X chromosome is 3x larger than the Y
& has many more genes
• Since females have 2 X
chromosomes, they can be carriers
for sex-linked traits
• Sex-linked disorders are more
common in males since they only
have one X chromosome
Pedigree
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Circle = female
Square = male
Shaded = person with trait
Unshaded = person without trait
Half shaded = carrier for a genetic disorder
Horizontal line = marriage
Vertical line & bracket = offspring
For some individuals in a pedigree, you can
determine genotypes based on their
phenotypes
Pedigrees: Tracing Autosomal
Traits
• If trait is recessive, shaded
individuals would be nn, unshaded
would be NN or Nn, carriers would
be Nn
• If trait is dominant, unshaded would
be nn, shaded would be either NN
or Nn
Pedigrees for sex-linked traits
• Shaded females must be
homozygous recessive XnXn
• Unshaded females could be XNXN or
XNXn - carrier
• Shaded males must have the
recessive allele XnY
• Unshaded males must have the
dominant allele XNY
Karyotype
• Picture of all the chromosomes in a cell
(see pg. 217)
• Can show gender as well as any
chromosomal abnormalities
– Down syndrome: extra chromosome #21
– Turner’s syndrome: female only has 1 X
– Klinefelter’s syndrome: male has XXY
– Caused by non-disjunction, which is when
chromosomes fail to separate during
meiosis