Bellwork There have been two main hypotheses to
Bellwork There have been two main hypotheses to how traits are passed onto offspring. The first is the “blending” hypothesis, where traits are mixed to form an intermediate trait. An example would be red and white flowers mate to form pink offspring. The other hypothesis is the “particulate” hypothesis where parents pass on discrete heritable units, an example of this would be if red and white flowers mate to make some offspring that are red and some that are white. • Which do you think is correct? Support your argument with evidence.
More Bellwork Define in your own words • • Homozygous Heterozygous Phenotype Genotype
Learning goals Who is Mendel? Know how to put together a punnett square with a monohybrid and dihybrid cross. Know what is meant by the law of segregation and the law of independent assortment. Know why you would use a testcross.
Who is Mendel A monk living in the 1800’s Interested in the causes of variation in plants – used peas to study inheritance Why Peas? • Short generation time • Large number of offspring • Could control mating
Mendel’s Findings How did he discover “genes” when there was not the technology to look at chromosomes under the microscope? How did he know that we get one “chromosome” from each of our parents? • Note: he did not call them genes or chromosomes, but rather, a heritable factor
Mendel’s Findings
Mendel’s Findings 1. Alternative versions of genes account for variations • Ex. Two versions of flower color – purple and white 2. Organisms inherit two alleles – one from each parent 3. For one trait, the dominant allele determines the appearance over the recessive allele 4. Law of segregation: two alleles separate during gamete formation
Punnett Squares
Punnett square practice Red flowers are dominant over white flowers. What is the probability that two flowers, both heterozygous for flower color, will have heterozygous offspring? What is the probability of a homozygous dominant and a heterozygous flower having offspring that is white?
Purple flower either PP or Pp. How can we determine its genotype?
Use punnett squares to determine phenotype Testcross – use a homozygous recessive individual to determine if an individual is homozygous dominant or heterozygous.
Mendel’s findings with two traits Monohybrid cross – a cross that creates heterozygosity for one character • EX round seeds RR x rr Dihybrid cross – a cross that creates heterozygosity at two characters • Ex round seeds and seed color YYRR x yyrr
Law of independent assortment Genes segregate independent of one another (as long as they are on different chromosomes)
Dihybrid crosses A pea plant heterozygous for flower position and stem length (Aa. Tt) are allowed to self pollinate. Axial is dominant over terminal and tall is dominant over dwarf. Draw a punnett square for this cross. How many offspring would be predicted to have terminal flowers and be dwarf?
Now try one on your own If a pea plant heterozygous for flower color and seed shape Pp. Rr is crossed with a pea plant heterozygous for flower color and homozygous recessive for seed shape (Pprr), what are the phenotypic ratios?
Multiplication rule Simply multiply the chances Example with the lottery: Winning the lottery • 1/18 million Winning the lottery twice • 1/18 million x 1/18 million = 1/324 trillion
Try it with dihybrid crosses A pea plant heterozygous for flower position and stem length (Aa. Tt) are allowed to self pollinate. Axial is dominant over terminal and tall is dominant over dwarf. How many offspring would be predicted to have terminal flowers and be dwarf? ¼ x ¼ = 1/16
More practice The genotype of F 1 individuals in a tetrahybrid cross is Aa. Bb. Cc. Dd. Assuming independent assortment of these four genes, what are the probabilities that F 2 offspring will have the following genotypes? (Hint: use the probabilities in a monohybrid cross and then multiply them. ) A. aabbccdd B. Aa. Bb. Cc. Dd C. AABBCCDD
Exceptions to Mendelian genetics Incomplete dominance – neither allele is completely dominant over the other allele • Ex. Red snapdragons crossed with white snapdragons make pink offspring
Exceptions to Mendelian genetics Codominance – Both alleles affect the phenotype • Ex. ABO blood typing
Inheritance cont Pleiotropy – One gene having many phenotypic effects • Ex. Multiple symptoms for one genetic disease Epistasis – A gene at one locus affects a gene at another locus • Ex. Black coat color in mice is dominant to brown (Bb) A second gene deterimes whether or not pigment will be deposited into the hair – the dominant allele is coat color (Cc)
Inheritance continued Polygenic inheritance – Many genes contribute to the phenotype • Ex. Skin color in humans
Recessive Inherited Disorders “live” in a population because they are carried by heterozygotes • Ex. Cystic fibrosis Normal allele codes for a membrane protein that transports chloride ions If there is not a normal allele, causes mucus to build up in the lungs, pancreas, digestive tract and other organs One out of every 2, 500 people of European descent have it and one out of 25 people of European descent carry it
Dominantly Inherited Disorders can be due to dominant alleles. • Ex. Achondroplasia • If two people, heterozygous for Acondroplasia have children, what is the probability that they would be homozygous recessive (Normal size)
Pedigrees
Analysis of Pedigrees
Analysis of Pedigrees
Types of Inheritance Autosomal recessive Autosomal dominant Sex linked • X-linked recessive • X-linked dominant Mitochondrial
Sex Linked inheritance Males – XY Females – XX Y chromosome carries genes for sex determination X chromosome carries many other genes besides sex determination (such as colorblindness)
X – Linked recessive Affected males transmit the gene to all daughters but not to sons Affected females will always produce affected sons Incidence in males higher than in females
X-linked Dominant Affected males produce all affected daughters and no affected sons
Now try to determine the type of Pedigree X linked recessive Autosomal dominant Mitochondrial
Genetic disorders caused by chromosome alterations Nondisjunction – homologous chromosomes or sister chromatids fail to separate during meiosis
Results of nondisjunction Aneuploidy – abnormal number of chromosomes Monosomic – missing a chromosome Trisomic – an extra chromosome • Ex. Down Syndrome (Trisomy 21) • Trisomy 13 Cleft palat Extra toes and fingers Decreased muscle tone Developmentally delayed
Polyploidy – more than two sets of chromosomes • Ex. Bananas are triploid (set of three chromosomes) and wheat is hexaploid (set of 6 chromosomes)
Alterations in chromosome structure
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