Download Heredity Chapter 5-2

DO NOW:
4/25
If you flip a coin what are the chances it will land on
heads? Tails? Suppose that you flipped the coin and
got heads. What are the chances that you will get
heads again?
Objectives:
1. Explain how genes and alleles are related to genotype
and phenotype.
2. Use the information in a Punnett square.
A Great Idea

 Mendel knew that there must be two sets of
instructions for each characteristic.
 The instructions for an inherited trait are called
genes.
 The different forms (often dominant and recessive)
of a gene are alleles.
A Great Idea, Continued

 Phenotype : An organism’s appearance.
 Genes affect the phenotype.
 Genotype: The combination of inherited alleles
together.
 A genotype that has 2 dominant OR recessive alleles is
homozygous or true breeding (PP or pp).
 A genotype that has one recessive and one dominant is
heterozygous (Pp).
A Great Idea, Continued

 Genotype vs. Phenotype
 An example of a
genotype that a
butterfly can inherit is
AA.
 The phenotype this
offspring will have is
dark wings.
 Punnett Squares are used to organize all the possible
genotype combinations of offspring from particular
parents. What are the genotypes? Phenotypes?

What are the Chances?

 A gene is made up of 2 alleles.
 Probability is the mathematical chance that
something will happen and is often written as a
fraction or percentage.
 Probability is used in genetics to figure out the
percentage of offspring that will have a particular
phenotype.
DO NOW:
4/26
In pea plants, the genes for tall plants (T) are
dominant over the genes for short plants (t).
Cross a homozygous dominant tall pea plant (TT)
with a short (homozygous recessive) pea plant
(tt).
What percentage of short pea plants will appear
in the offspring of this cross?
Objectives: (Do Not Have to Re Copy)
1. Explain how probability can be used to predict possible
genotypes in offspring.
2. Describe 3 exceptions to Mendel’s observations.
Calculating Probability

What are the Chances?
Continued

 Genotype Probability
 To have white flowers, a pea plant must receive a
lower case p allele from each parent.
 Each offspring of a Pp X Pp cross has a 50% chance of
receiving either a P allele or p allelle from the parent.
 So, the probability of inheriting two p alleles is 1/2 X
1/2, which equals 1/4, or 25%.
Mendel’s Principles

 Mendel came to 3 important conclusions from his
experimental results:
1. The inheritance of each trait is determined by “units”
or “factors” that are passed on to descendants (these
units were later called genes)
2. An individual inherits one such gene from each
parent for each trait.
3. A trait may not show up in an individual but can still
be passed on to the next generation.
More About Traits
(Exceptions to Mendel’s principles)

 Incomplete Dominance
 Researchers have found
that sometimes one trait
is not completely
dominant over another.
 When a red and white
snapdragon flower breed
it makes offspring that
are all pink.
 Example in humans:
Wavy hair
More About Traits
(Exceptions to Mendel’s principles)

 One Gene, Many Traits
 Sometimes one gene influences multiple traits.
 Example: A gene for fur on a tiger to be white may
also code for the tiger’s eyes to be blue.
 Many Genes, One Trait
 Some traits, such as the color of your skin, hair,
and eyes, are the result of several genes acting
together. This is called multiple alleles.
 Examples: Eye color
More About Traits,
Continued

 The Importance of Environment
 Genes aren’t the only influences on traits.
 A combination of things determine an individual’s
characteristics.
 Example: Diet affects growth and sun affects skin tone
 Lifestyle choices can also affect a person’s traits.
1. What is the genotype of the offspring
represented in the upper left-hand box of
the Punnett square?

A RR
B Rr
C rr
D rrr
2. What is the genotype of the offspring
represented in the lower right-hand box
of the Punnett square?

A RR
B Rr
C rr
D rrr
3. What is the ratio of Rr (purple-flowered
plants) to rr (white-flowered plants) in the
offspring?

A 1:3
B 2:2
C 3:1
D 4:0