Genetics and Heredity

What is heredity?

• The passing on of characteristics from parents to offspring.
• Genetics is the branch of biology that studies heredity
• the characteristics that are inherited are traits.

Father of Modern Genetics

• Gregor Mendel – Augustinian Monk in 1856 to 1863
• Order of monks who believed in working for their living and learning, each one given a task in the Monastery and could pursue their own studies while doing so
• Experimented on heredity and traits
• First person to be able to predict traits

Mendel’s Subject

• He chose pea plants to study originally, which reproduce sexually, which means they have two distinct sex cells, male and female.
• sex cells are called gametes.
• the transfer of male pollen grains�to the pistil of a flower is called�pollination
• the uniting of male and female �gametes is called fertilization

Mendel’s Subject

• With the Pea plants Mendel compared several traits
• Height of plant
• Color of flower
• Color of seed
• Shape of seed
• Location of flower

Mendel’s Subject

• He directly controlled the reproduction of the flowers
• By doing so he was able to see what�would happen if he were to breed a�tall plant with a short plant
• Breeding two organisms with�different traits is called crossing

Crosses

• A hybrid is the offspring of parents that have different forms of a trait, such as tall and short.
• Mendel first did monohybrid crosses, which means he was looking at only one trait.
• He later did dihybrid and larger crosses but we will focus on monohybrid crosses

Mendel’s crosses

He bred all of the�traits to the right and�with each trait he �found a similar �pattern as is shown �in the left picture. Tall�and short plant have �all tall offspring,�breeding two of the offspring you get a ratio of 3 tall to every one short.

The rule of dominance

• Every trait is�coded for by�two genes
• the genes �have different �forms called �Alleles.
• Alleles are �either dominant, recessive or co-dominant

The rule of dominance

• Dominant traits�overpower a�recessive trait,�we say the trait�shown is being�expressed
• organisms that �have even one�dominant allele it will be expressed over any recessive ones.
• Dominant traits are written with a uppercase letter

the law of segregation

That when gametes are formed, the parent separates their two genes that code for the same trait. So the child gets one from each parent. The parent cannot control this.

phenotype

• the way an organism looks and behaves is called it’s phenotype.
• example: what color are your eyes, or your hair?
• what you see is the phenotype.

Genotype

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• the gene combination an organism contains is known as genotype. So in the example above TT is a genotype where tall is a phenotype

Homozygous vs. Heterozygous

• an organism that is homozygous for a trait has two alleles for the trait that are the same.
• an organism that is heterozygous for a trait has two different alleles (dominant and recessive).

Punnett Square

What is a Punnett Square

• A chart that we use to determine the different genotypes that offspring can inherit from their parents
• Important! It doesn’t say what WILL happen rather what CAN happen

Punnett Square

Putting it into practice

Lets find out if the two pigeons on the left produce offspring that have the traits of the pigeon on the right

Punnett Square

How to use a Punnett Square

• First we must get the genotype of each parent. (usually only available through observation of multiple generations)

Example

• Z: allele for gray feathers�z: allele for white feathers

Zz – mother’s genotype Zz father’s genotype

Place the Parent Genotypes�it doesn’t matter which parent goes where

Z

Z

z

z

Drop the alleles from the top into each space beneath them, Dominant alleles are written before recessive ones

Z

Z

z

z

Z

Z

z

z

Slide the alleles from the other parent into the spaces next to it

Z

Z

z

z

Z

Z

z

z

Z

Z

z

z

Reading a Punnett Square

Z

Z

z

z

Z

Z

z

z

Z

Z

z

z

• First we must pull out the possible genotypes
• Take the two letter combinations from each box and list them out
• ZZ, Zz, Zz, zz

Reading a Punnett Square

• ZZ, Zz, Zz, zz are the possible genotypes
• Now we need to determine how probable each possibility is.

Probability

• What is probability?
• Probability is the chance that an event will occur
• How do you find the probability of an event?
• You must know:
• The number of specific events (like a specific number on a dice)
• The number of possible events (how many numbers are on the dice)

Probability and Punnetts

• Looking at our possible Genotypes we have ZZ, Zz, Zz, zz
• The number of possible events in a monohybrid cross is always 4. (there were 4 boxes, so 4 options)
• Let’s choose a specific trait

Probability and Punnetts

• Looking at our possible Genotypes we have ZZ, Zz, Zz, zz
• Let’s choose gray feathers (Z)
• We have 3 events of gray feathers

Probability and Punnetts

• Probability =

Number of specific event 3 reduce if necessary�Number of possible events 4

• The probability of having a gray offspring is ¾, 3:4 or 75%

White Offspring

• This leaves a 1/4 chance of having a white offspring
• So yes two gray pigeons can have a white offspring
• Also if you are still confused feel free to look up videos on how to use Punnett squares on YouTube, there are tons.