Gregor Mendel and Genetics

Gregor Mendel was a Swiss Monk who studied genetic traits in Pea

Plants

Inheritance

• We each inherit 2 forms of each gene – 1 from each parent (from sperm and egg)– The genes code for the same traits (eyes,

ears, fingers) but variations of those traits– Blue or Brown eyes, hairy knuckles, non-hairy

knuckles, sickle cell /non-sickle cell– These variants are called alleles

Mendel’s “Laws”• Law of Independent Assortment

– Genes are usually inherited independent of each other (so you can be tall and blond, etc)

– Mendel used Pea plants – he saw that each trait (flower color, height, seed color, seed shape) were all inherited separately from each other

• Law of Segregation – The genes are separated into gametes (sex cells) and reunited during fertilization

Types of Inheritance

• Complete dominance:– One allele is dominant over the other –

inheriting only one dominant allele will show this trait

– The other allele is considered recessive – one needs to inherit this variant from both parents

Other Inheritance patterns• Incomplete dominance – blending of

genetics – black cat + white cat = gray cat

• Co-Dominance – neither gene is completely dominant – blood typing – type A, B, AB or i or type O

• X- linked – traits carried on the X chromosome tend to be seen in men who have only 1 x-chromosome

Eye color

• Eye color is a complete dominance inheritance pattern:

• Brown is represented by B

• Blue is represented by b

• The genotype of a person with brown eyes could be: BB or Bb– One “B” from each parent and – a second “B” or ‘b’ from each parent

Heterozygous and Homozygous

• If both copies of the allele are the same, one is considered to be homozygous for that trait (BB OR bb)

• If one has two different alleles for a gene one is considered to be heterozygous for that trait (Bb)

Punnett Squares

• A heterozygous parent has genotype:

• Bb

• A parent homozygous for blue eyes has genotype:

• bb

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Genetics is about Probabilities

• Bb x bb• Look at the boxes that

show all of the possible results:

• BB = • Bb/bB =• Bb =• Genotype – what their

allele distribution is:

• Phenotype probabilities- what they will look like:

Heterozygous black guinea pigs are mated to homozygous white guinea pigs. Do the punnett square for this cross. (Use B for black and b for white)

Genotype Ratios

% BB

%Bb

%bb

Phenotype Ratios

% Black

% White

Genetics Is Probabilities

• Each mating is a new “throw of the dice”

• Every time it is the same

• Women are XX• Men are XY• XY x XX

• What are the chances of having a boy?

Seed Shape

Pod Shape

Monohybrid Crosses

• When we look at the inheritance patterns of a single gene we call this a monohybrid cross

• When we look at the inheritance patterns of two genes at the same time we call this a dihybrid cross

Dihybrid Crossing

• What happens when you look at 2 different traits?

When Mendel looked at the inheritance of two separate traits, he discovered that the outcome for each trait was unchanged from his examination of individual traits. He determined that most traits are inherited independently of others. This became his Principle of Independent Assortment

There are 4 different ways that the alleles for the seed color and seed shape can be combined.

These 4 possible combinations can result in 16 different genotypes

Of the sixteen possible outcomes: How many are round and yellow?

How many are wrinkled and yellow?

How many are round and green?

How many are wrinkled and green?

Which color is dominant? __________

Which shape is dominant? ________________________

X- linked traits

The X carries more information than the Y-Chromosome

Females are XX

Males are XY

Therefore, any additional information on the male’s X chromosome will show, even if it is recessive

X-Chromosome/ Y Chromosome

How we indicate a X-linked trait

• We use a regular X with a subscript

Indicator letter:

Hemophilia is indicated

As

Xh - it is recessive

XhY – This male will have hemophilia

Xh X – This female will not have hemophilia

Hemophilia – X-linked trait

• Hemophilia is a disease of bleeding.

• Why is it called an “X-linked” trait?

• Xh – to show that the gene for hemophilia is on the chromosome.

Carriers

• Hemophilia is a recessive gene – it needs to be on both X chromosomes for a female to have the disorder. If only 1 chromosome has it she is a Carrier

• We indicate a carrier this way:

• XXh – Notice only 1 of the X chromosomes carries the disorder

• XhXh - Female who HAS the disorder

Females are Carriers

• Why can’t males be carriers?

• We indicate a male with hemophilia this way:

• XhY

Punnett squares A female carrier and

a normal male:

Female carrier

______

Normal Male

_________

Phenotype Ratios:

• Affected Males: Female carriers

• Affected Females• Genotype Ratios:• XY _____

Xhx _____• XX_____

XhXh___

Probability of having a child with hemophilia