genetics. study of heredity why do cats have kittens and not puppies? why do humans give birth to...
TRANSCRIPT
Study of Heredity
• Why do cats have kittens and not puppies? Why do humans give birth to other humans? What controls what your child will look like?
• Genetics: the study of heredity
• Heredity: passing of traits from parents to offspring.
Imagine a time before we knew of DNA and genes. These things
were a great mystery.
Original Ideas in Genetics
• Blended Inheritance:– Mother’s traits and
Father’s traits are blended to make the child’s traits.
• A strong man and a weak woman would have a child with average strength.
• We know this is not true today, because of Gregor Mendel.
Gregor Mendel
• Born in Austria in 1822
• Became a monk• Worked in the
monastery garden• Was interested in
science• Began to experiment
with peas.
Mendel cross-fertilized peas
• Why peas?– Reproduce really quickly– Could control the matings of pea plants– Peas are Hermaphrodites– Grows easily and produces MANY offspring
What did he do?• Self-pollinated two
different plants
• This made each plant true-breeding.
• Finally, he had two separate true-breeding plants that he could cross-pollinate.
Vocabulary• Self-pollination-letting the male part of one plant
fertilize the female part of the same plant. (Hermaphroditic part) AKA:inbreeding
• True-breeding-a plant that produces offspring that display only one form of a trait. (Ex- purple plant makes purple plants)
• Cross-pollination-letting the male part of one plant fertilize the female part of a different plant (more like dating)
What traits did Mendel look at?
• Seed shape
• Seed color
• Pea Pod shape
• Pea Pod color
• Flower color
• Flower position
• Plant size
Mendel had specific names for each generation of plants.
• P generation– Parental generation, the first two individuals
that are crossed in a breeding experiment.
• F1 generation
– First filial generation, the offspring of the P generation
• F2 generation
– Second filial generation, the offspring F1
generation
As Mendel worked, he discovered…
• Blending inheritance was not correct!
• Wrinkled + smooth peas =– No half-wrinkled peas!!!
– Only fully wrinkled or smooth peas.
– With continued work, he discovered…..
Mendel’s Four Hypotheses• Mendelian Theory of Heredity
– 1. When gametes are formed, only one allele for each gene goes into each gamete. This happens during Meiosis II.
• There’s only one allele in each sperm and only one allele in each egg.
– 2. An individual gets two genes for each trait (one from each parent)
• Each pea plant gets one gene for pod color (one from each parent plant)
– 3. These two genes are called Alleles and there are different versions of alleles
• For the pea pod color trait: yellow or green or white, etc.
– 4. Of these different versions of alleles, one is dominant and one is recessive
• For the pea pod trait : Green alleles are dominant, yellow alleles are recessive
What does this mean? Example:• You have two genes (alleles) for
each trait. 1. Your Gametes have half of your chromosomes (only 1 of the pair of allele for each trait).
• 2. At fertilization, the embryo gets one allele from each parent. In this case, you get Mom’s allele for wrinkles and Dad’s allele for smoothness (3. There are different versions of alleles).
• 4. Of the different versions of alleles, one is dominant and one is recessive.
• Dominant = smoothness Recessive = wrinkled
What do you mean by dominant and recessive?
• Example:• 1 allele for wrinkles
(recessive)• 1 allele for smoothness
(dominant)• You have 1 dominant and 1
recessive allele. The dominant shows through, masking the recessive allele.
• You appear smooth!
• The dominant allele always shows through, masking the recessive allele.
• The recessive allele only shows through when there is no dominant allele present.
We represent alleles with letters.• Example: • Trait: Pea pod color• Alleles: 1 wrinkled, 1 smooth• Smooth = dominant• Wrinkled = Recessive• **Dominant alleles are written with capital
letters and Recessive alleles are written with lower-case letters.
• Ss• Will this pea appear smooth or wrinkled
and how do you know?• Smooth, because the dominant allele will
mask the recessive allele.
• Another One:• Trait: Eye color• Alleles: 1 blue, 1
brown• Brown = dominant• Blue = Recessive• Bb• What color eyes will
this person have?• Brown
A couple more….
• Trait: Eye color• Alleles: 2 blue• Brown = dominant• Blue = Recessive• bb• What color eyes will
this person have?• Blue!
• Trait: Freckles• Alleles: 1 freckles, 1
no freckles• No freckles =
dominant• Freckles = Recessive• Ff• Does this person have
freckles?• Yes
Some vocabulary
• Try identifying the following combinations of alleles:
• Tt heterozygous• jj Homo. Recess.• Ll heterozygous• MM Homo. Dominant
• Homozygous Dominant– Both alleles are dominant, BB
(both are capital letters)
• Homozygous Recessive– Both alleles are recessive, bb
(both are small letters)
• Heterozygous– One allele is recessive, the other is
dominant
– Bb (one capital, one brown small)
– **Always write the dominant/capital letter first.
A few last terms!
• Genotype– Set of alleles that an individual has (letters)– Bb
• Phenotype– Physical appearance of the individual (what
you see)– Brown eyes
Determine the genotype of the following individuals.
• An alien has 1 allele for wings and 1 allele for no wings. (Wings are dominant)
• Ww• An alien has 2 alleles for striped fur. (Striped fur
is recessive)• nn• An alien has three eyes. (Three eyes is a recessive
trait)• ff
Determine the phenotype of the following individuals.
• An alien has 1 allele for wings and 1 allele for no wings. (Wings are dominant)
• It has wings.• An alien has 2 alleles for striped fur. (Striped fur is recessive)• It has striped fur.• An alien has a genotype tt. (t = allele for three eyes, which is
a recessive trait)• It has three eyes.• An alien with the genotype Rr. (R=red eyes, r=purple eyes)• It has red eyes.
Understanding how traits are inherited is important!
• Traits like eye color and hair color, but also heritable diseases like Cancer.
• So scientists want to know the probability of passing on these diseases. To calculate probabilities, geneticists use Punnett squares.
Let’s say that you and your husband go to a geneticist and discover that you both are
heterozygous for Huntington’s Disease! What are the chances of your children inheriting the
disease?Mom’s gametes
Dad’s gametes
Dad’s genotype: Rr
Mom’s genotype: Rr
• 1 out of 4 will be homozygous dominant (RR)• 2 out of 4 will be heterozygous (Rr)• 1 out of 4 will be homozygous recessive (rr)• Which genotype will result in Huntington’s Disease?
– The “rr” genotype only!
• So your chances of having a child with Huntington’s Disease is ¼ or 25%.
Try one on your own!
Cross a mother with the genotype Dd and a father with the genotype DD. What are the
chances that their child will have the genotype DD, like Daddy?