Download 2/25/02 Lecture Highlights: Inheritance

2/25/02 Lecture Highlights: Inheritance
Inheritance - Outline
• Human phenotypes
• Genotypes (dominant and recessive)
• How are they inherited?
– Mendel’s law of segregation
– Mendel’s law of independent assortment
Can you roll your tongue?
Clasp your hands together, interlacing your fingers… Which thumb is on top- left or right?
Widow’s peak – dominant trait
Detached earlobes – dominant trait
Attached earlobes – recessive trait
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Leonardo DiCaprio and Parker Posey’s phenotypes are detached earlobes/ widow’s peak
Matt Damon’s phenotype is attached earlobes/ widow’s peak
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So, phenotype is a description of physical appearance
Genotype is the genetic basis for appearance (and other unseen traits)
What do dominant and recessive mean?
First we need to look at a cell….
• Chromosomes are found in the nucleus
• Human chromosomes are in pairs (we are diploid- see glossary)
• One pair of chromosomes – homologous
• locus – location of a particular gene on a chromosome
• allele – different forms of a gene at a locus
• homozygous - 2 copies, same allele (e.g., blue/blue)
• heterozygous -2 copies, diff. alleles (e.g., red/green)
Let’s say red allele means detached earlobes and green allele means attached
9 Remember… attached earlobes are dominant trait
9 So, the person’s genotype is red/green, BUT – their phenotype is detached earlobes
ƒ green is masked!
This locus codes for hairline (widow’s peak = dominant; straight hairline = recessive)
9 Now let’s say the blue genotype means straight hairline phenotype
9 This person will have straight hairline (results only when both alleles are blue)
•
dominant allele –one dominant allele present in the genotype = automatically expressed in the phenotype
(masks recessive)
• recessive allele – both copies on homologous chromosomes must be same for recessive to be expressed in
phenotype
Inheritance
We get 1 set of chromosomes from mom & 1 from dad
• I can roll my tongue (dominant) but my mom can’t (recessive) –
• let’s say now R is the allele for rolling and r is allele for not
• Can my dad roll his tongue?
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What’s my mom’s genotype?
What’s my dad’s genotype?
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Can my sister tongue roll?
Let’s say dad is heterozygous…(Punnett square)
Dad’s
sperm
R
r
Mom’s eggs
r
r
Rr
Rr
rr
rr
Possible offspring genotypes (bold)
Bird breeding example…
Gregor Mendel (1822-1884)
• Augustinian monk (Austria)
• Studies on plant hybridization/ inheritance
• Laid foundation for modern genetics
• Results published in 1865
– BUT ignored until 1900
Mendel’s first experiment – pea plants with purple and white flowers
1. Removed stamens from purple flower (prevent self-fertilization)
2. Transferred pollen from white to purple flower
3. plants made seeds
4. planted seeds and observed offspring (F1) phenotypes
5. Bred F1s together
6. observed F2 (“grandchild”) phenotypes
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Results: F1 generation all purple; F2 generation ¾ purple, ¼ white (purple dominant, white recessive)
These experiments resulted in Mendel’s principle of segregation
– pairs of alleles segregate (separate) during gamete formation (when eggs and sperm are made);
fusion of gametes at fertilization creates allele pairs again
Mendel looked at many phenotypic traits in pea plants
• Through other crosses- determined yellow dominant to green and round peas dominant to wrinkled
Mendel’s second experiment (dihybrid cross; 2 traits – pea color and pea shape)
• F1 – all yellow, round (dominant traits)
• F2 – not 3:1 ratio, but 9:3:3:1 ratio
o He knew something other than segregation was going on….
• Principle of independent assortment
o Each pair of alleles segregates independently of one another during gamete formation
Glossary of genetics terms:
genotype – genetic type (what genes a person has for a particular trait; usually represented by 2 alleles; for example
genotype Ff)
phenotype – physical appearance (influenced by one or many genes; for example a person with genotype Ff or ff has
freckles, which is their phenotype)
gene – segment of DNA that codes for a particular protein
allele – different forms of a gene
chromosome – tightly wound up DNA that contains many loci (and therefore many genes)
homologous chromosomes – pairs of chromosomes that contain the same genes (inherit one from mom and one from
dad)
diploid – 2 copies of chromosomes (and hence the genes on them)
homozygous – 2 copies of the same allele (e.g., FF or ff) on homologous chromosomes
heterozygous – different alleles on homologous chromosomes (e.g., Ff)
Punnett square –determines offspring genotypes by matching up parental genotypes
Genetics Concepts
dominant – if one dominant allele is present in the genotype, it is automatically expressed in the phenotype although
there are 2 copies in diploid organisms; it masks the other allele on the other chromosome); Freckles are
a dominant trait, so if a person has either genotype FF or Ff, they will have the freckle phenotype.
recessive – for a recessive allele to be expressed, it has to be present on both chromosomes. No freckles are
recessive, so a person must have the genotype ff to have no freckles as their phenotype.
codominant – both copies are expressed (e.g., “red” gene on one chromosome and “white” gene on another
chromosome -> genotype = red/white; but phenotype = pink). A plant with codominant flower color
genes would have red flowers if the genotype is RR, white flowers if the genotype is WW and pink
flowers if the genotype is RW