Genetics and heredity
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Genetics and Heredity. Ch.16-1. Genes and Alleles. Chromosomes contain DNA Sections of DNA are called genes You have a gene that determines every trait (characteristic) you have Each gene has a pair of alleles (two) The alleles control what trait that gene expresses.

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Genetics and Heredity

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Genetics and heredity

Genetics and Heredity

Ch.16-1


Genes and alleles

Genes and Alleles

  • Chromosomes contain DNA

  • Sections of DNA are called genes

  • You have a gene that determines every trait (characteristic) you have

  • Each gene has a pair of alleles (two)

  • The alleles control what trait that gene expresses


Dominant and recessive alleles

Dominant and Recessive Alleles

  • Basic traits have two alleles, one dominant allele and one recessive allele.

    • Ex) eye color in a population of bunnies

      • Brown eyes are dominant

      • Blue eyes are recessive

  • We represent each allele with a letter.

  • In most cases, the dominant allele is a capital letter and the recessive allele is a lowercase letter.

    • Ex) Brown = B, Blue = b


Dominant and recessive alleles1

Dominant and Recessive Alleles

  • A dominant allele will be expressed (shown) over a recessive allele.

    • Ex) Possible combinations of alleles for bunny eye color:

      • BB  brown eyes – both alleles code for brown eyes

      • Bb  brown eyes – the brown allele dominates the blue allele and the bunny has brown eyes

      • bb  blue eyes – both alleles code for blue eyes


Homozygous and heterozygous

Homozygous and Heterozygous

  • Combinations of the same allele are called “homozygous”.

  • Combinations of different alleles are called “heterozygous”

    • Ex) Possible combinations of alleles for bunny eye color:

      • BB  homozygous dominant

      • Bb  heterozygous

      • bb  homozygous recessive


Genotype and phenotype

Genotype and Phenotype

  • The specific combination of alleles is called the genotype. Genotype refers to all hereditary information whether it is expressed or not.

  • The outward appearance of a trait is called the phenotype. The phenotype refers only to what is observable about the trait.

  • Ex) Bunny Eye Color

    • Genotype = Bb, Phenotype = brown eyes


Heredity

Heredity

  • When a parent cell undergoes meiosis, the alleles separate into different sex cells.

  • The trait that gets passed on to the offspring (baby) depends on which pair of alleles the offspring happens to get.

  • Gregor Mendel pioneered the science of predicting which traits would be passed on from one generation to the next.


Gregor mendel

Gregor Mendel

  • Failed out of college

  • Became a monk

  • Studied pea plants

    • When certain plants are bred…

      • What is the shape of the pea?

      • What is the color of the bloom?

      • How tall is the stem?

  • Mendel noticed patterns between parent plants and offspring plants and developed these patterns into theories about heredity.


Punnett squares

Punnett Squares

  • We use Punnett squares to help predict the genotypes of offspring.

  • A punnett square requires that you know the genotype of each parent.

    • Ex) Mother = BB (brown eyes), Father = bb (blue eyes)

  • The parent genotypes are put along the sides of a square cut into four segments.

B

B

Mother

b

Father

b


Punnett squares1

Punnett Squares

  • Then, the alleles from the mother are brought down and the alleles from the father are brought over.

  • Each of the four combinations of alleles equals a possible genotype for the offspring.

In this case, all of the possible combinations of alleles (100%) are “Bb” and the offspring will have brown eyes.

Mother

B

B

b

B

b

B

b

brown eyes

brown eyes

Father

b

B

b

B

b

brown eyes

brown eyes


Punnett squares2

Punnett Squares

  • What if the parents had different genotypes?

    • Ex) Mother = Bb (brown), Father = Bb (brown)

In this case, the offspring has a 75% chance (3/4) of having brown eyes and a 25% chance (1/4) of having blue eyes.

Mother

B

b

B

B

B

b

B

brown eyes

brown eyes

Father

B

b

b

b

b

brown eyes

blue eyes


Punnett squares3

Punnett Squares

  • Try this combination on your own.

    • Ex) Mother = bb (blue), Father = Bb (brown)

In this case, the offspring has a 50% chance (2/4) of having brown eyes and a 50% chance (2/4) of having blue eyes.

Mother

b

b

B

b

B

b

B

brown eyes

brown eyes

Father

b

b

b

b

b

blue eyes

blue eyes


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