Transcript Introduction to genetics

INTRODUCTION TO
GENETICS
The Work of Gregor Mendel
• What organism does Mendel use to study genetics and heredity?
• Fertilization occurs when a sperm cell unites with an egg cell.
• Another word for true-breeding is self-pollination. How does self-pollination
occur in plants?
• He discovered the principle of dominance which states that some alleles are
dominant while others are recessive.
• If an allele is dominant, what does this mean?
• It means that it will always mask a recessive gene if a recessive gene is
present.
• EXAMPLE: T = tall pea plant (dominant) while t = short pea plant (recessive).
Alleles will possess two of these genes. Some will have Tt, TT and tt.
• Genetics is the study of heredity.
• A trait is a characteristic that varies from one individual to another. For
example, in Mendel’s study, seed color or seed shape.
• A hybrid is the offspring between parents with different traits.
The Work of Gregor Mendel
• Alleles are different forms of a gene.
• Genes are chemical factors that determine traits.
• Alleles segregate from each other and each gamete carries a single copy
of each gene. This is the law of segregation – the second law Mendel
observed during his pea plant experiments.
• TT is homozygous (“homo” = same; “zygous” = zygote) dominant. TT is
the genotype while the phenotype exhibits a tall plant.
• Tt is heterozygous (“hetero” = different; “zygous” = zygote). Note that
heterozygous alleles can neither be dominant nor recessive because
they contain both alleles. Tt is the genotype while the phenotype will be
the dominant because it will mask the recessive trait so the phenotype
will be a tall plant. HOWEVER, this hybrid will carry the recessive gene!
• tt is homozygous recessive. tt is the genotype while the phenotype will
exhibit a short plant.
The Work of Gregor Mendel
• What does segregation mean?
• What are gametes?
• Give examples of a gamete.
Probability and Punnett Squares
• When Mendel crossed two hybrid plants, 75% of the offspring were
tall while 25% of the offspring were short.
• Probability is the likelihood that a particular event will occur.
• Geneticists use probability to predict the outcomes of genetic
crosses.
• A Punnett Square is a tool used by geneticists to show the
probability of each possible outcome for offspring genotypes.
• Genotype = TT, Tt, or tt
• Phenotype = Tall (TT or Tt) or Short (tt). Remember, the dominant
allele (or form of a gene) will always mask a recessive allele if the
gene is heterozygous.
• https://youtu.be/u1TXhYOAWJ4
Probability and Punnett Squares
Probability and Punnett Squares
Probability and Punnett Squares
Probability and Punnett Squares
Which allele is dominant?
Exploring Mendelian Genetics
• The principle of independent assortment – genes segregate
independently during gamete formation; this explains genetic variations
in plants, animals and other organisms.
• Some alleles are neither dominant nor recessive, and many traits are
controlled by multiple alleles or multiple genes.
• Incomplete dominance - one allele is not completely dominant over
another. When a cross between a red flowered plant and a white
flowered plant produces pink flowers as a phenotype is an example.
• Codominance – both alleles contribute to the phenotype. When a black
chicken crosses with a white rooster and the offspring is “erminette” or
speckled black and white.
• Multiple alleles – many genes have more than two possible alleles.
• Polygenic traits – traits controlled by two or more genes.
Meiosis
• It is the process of producing gametes.
• 2N = diploid, which means, two sets. A cell that contains two sets of
homologous chromosomes is called diploid.
• 1N = haploid, which means, one set. A cell that contains a single set of
genes is called haploid.
• In animals, every cell in the body is diploid EXCEPT gametes. Gametes
are haploid.
• Meiosis is a reduction division in which the number of chromosomes per
cell is cut in half through the separation of homologous chromosomes in
a diploid cell.
• There are two phases of meiosis: meiosis I and meiosis II.
• During prophase I, tetrad’s are formed. Tetrad’s are 4 chromatids
paired together. When tetrads exchange portions of their chromatids,
this is called crossing-over.
Meiosis
Meiosis
Meiosis
Linkage and Gene Maps
• Chromosomes assort independently – but individual genes do not.
They are said to be “linked”. Genes on a chromosome that are
separated by large distances assort independently; those separated
by short distances are linked and do not assort independently.
• Crossover events during meiosis occasionally separate and
exchange linked genes to produce new combinations of alleles.
These events generate genetic diversity.
• Gene maps use recombination frequencies of traits to show relative
locations of genes. If two genes are close together, recombination
frequencies are low. If two genes are far apart, recombination
rates are high.
Chapter 11 Quiz
• 1. What is the scientific study of heredity?
• 2. What is a zygote?
• 3. What is a gene?
• 4. What is an allele?
• 5. If yellow seed color, Y, is dominant to green seed color, y, what
phenotype will a pea plant exhibit if its genotype is yy? Yy? YY?
• 6. If two hybrids of the above gene were crossed Yy x Yy, what would
the ratio of phenotypes be? Of genotypes?
• 7. Use the above symbols to write a heterozygous genotype. Do the
same and write two homozygous genotypes.
• 8. If a plant is said to exhibit incomplete dominance and a true-breeding
red flowering strain is crossed with a true-breeding white flowering
strain, what color will their offspring exhibit?
Answers
• 1. Genetics
• 2. A fertilized egg cell (sperm + egg)
• 3. Chemical factor that is passed down from parent to offspring
that can determine specific expressed traits (phenotypes)
• 4. A certain form of a gene
• 5. yellow ; green ; green
• 6. 3:1 – three green and one yellow ; 1:2:1 – one YY, two Yy, one yy
• 7. Yy ; yy & YY
• 8. pink