The Basics of GeneticsThe Basics of Genetics

6 Laws about Heredity6 Laws about Heredity

1. Traits are passed or inherited 1. Traits are passed or inherited from one generation to the next.from one generation to the next.

• Your genetic instructions are located in the nucleus of Your genetic instructions are located in the nucleus of every cell, stored within every cell, stored within chromosomeschromosomes..

How will your traits be passed on to How will your traits be passed on to your future children?your future children?

• Fathers pass genetic Fathers pass genetic instructions to instructions to offspring through offspring through spermsperm cells. cells.

• Mothers pass genetic Mothers pass genetic instructions to instructions to offspring through offspring through eggegg cells.cells.

2. Genes control the traits of an 2. Genes control the traits of an organism.organism.

•No, not No, not THAT THAT Gene!Gene!

2. Genes control the traits of an 2. Genes control the traits of an organism.organism.

• A gene is a section of A gene is a section of a a chromosomechromosome, that , that codes for a codes for a specific specific traittrait..

• ChromosomesChromosomes are are made of tightly wound made of tightly wound strands of strands of DNADNA

3. Organisms inherit genes 3. Organisms inherit genes in pairsin pairs, , one from each parent.one from each parent.

• Human sex cells (sperm Human sex cells (sperm or egg) contain or egg) contain 2323 chromosomes each. chromosomes each.

• When the egg is When the egg is fertilized, the embryo fertilized, the embryo will have will have 46 46 chromosomes in each of chromosomes in each of its cells, which is the its cells, which is the correct number of correct number of chromosomes for a chromosomes for a human.human.

Sexual Reproduction Sexual Reproduction (2 parents) creates unique offspring.(2 parents) creates unique offspring.

• In plants, male In plants, male chromosomes are chromosomes are passed through passed through pollenpollen grains to female grains to female egg egg cells. Once cells. Once fertilized, these eggs fertilized, these eggs will develop into will develop into seedsseeds..

• The new plants will The new plants will receive a combination receive a combination of traits from both of traits from both parents.parents.

4. Some genes are 4. Some genes are dominantdominant, , while others are while others are recessiverecessive..

• ““Stronger” traits are Stronger” traits are called called dominantdominant. .

• ““Weaker” traits are Weaker” traits are called called recessiverecessive..

• Geneticists use Geneticists use symbols symbols (letters)(letters) to to represent the different represent the different forms of a gene. forms of a gene.

DominantDominant traits are traits are represented by a represented by a capital lettercapital letter..

• Yellow seeds Yellow seeds are are dominant……..dominant……..

YY

RecessiveRecessive genes (for the same trait) genes (for the same trait) are represented by are represented by THE THE SAMESAME letter, but it is letter, but it is lower caselower case . .

Green seeds are Green seeds are recessive……….recessive……….

yyDominant (yellow) = YDominant (yellow) = Y

Recessive (green) = yRecessive (green) = y

• In pea plants, tall In pea plants, tall is dominant over is dominant over short. The letter short. The letter used to represent used to represent the the tall tall gene is gene is T T (BIG T)(BIG T). .

• The The shortshort gene is gene is represented by represented by t t (little t)(little t)..

• In pea plants, tall In pea plants, tall is dominant over is dominant over short. The letter short. The letter used to represent used to represent the the tall tall gene is gene is T T (BIG T)(BIG T). .

• The The shortshort gene is gene is represented by represented by t t (little t)(little t)..

Pea Plant Genotype

Tall =TTall =TShort = tShort = t

In humans, brown eyes are dominant In humans, brown eyes are dominant over blue. over blue.

Brown = BBrown = B

Blue = bBlue = b

5. 5. Dominant genes hide recessive genesDominant genes hide recessive genes when an organism inherits both.when an organism inherits both.

• Y + y = yellow seeds Y + y = yellow seeds

(yellow is dominant)(yellow is dominant)• T + t = tall plant (tall T + t = tall plant (tall

is dominant)is dominant)• B+ b = brown eyes B+ b = brown eyes

(brown is dominant)(brown is dominant)

A A PUREBREDPUREBRED organism has two of organism has two of the same genes for a trait. the same genes for a trait.

• TT = TT = purebredpurebred TALL TALL• tt = tt = purebredpurebred SHORT SHORT

• This genotype is also This genotype is also called called

HOMOZYGOUSHOMOZYGOUS

A A HYBRIDHYBRID organism has two organism has two different genes for a trait.different genes for a trait.

• Tt = hybrid TALL Tt = hybrid TALL (tall is dominant)(tall is dominant)

• This genotype is also This genotype is also called called

HETEROZYGOUSHETEROZYGOUS

Will a Will a hybridhybrid human with the genes human with the genes Bb have brown or blue eyes? Bb have brown or blue eyes?

• Bb = Brown eyes Bb = Brown eyes • The dominant gene (brown), will be expressed in a The dominant gene (brown), will be expressed in a

hybrid.hybrid.

Why must all blue eyed people be Why must all blue eyed people be PUREBREDPUREBRED for that trait? for that trait?

• If the dominant gene is present, it will always be If the dominant gene is present, it will always be expressed.expressed.

• The only possible gene combination for blue eyes is The only possible gene combination for blue eyes is

bbbb

6. Some genes are neither dominant 6. Some genes are neither dominant nor recessive. These genes show nor recessive. These genes show

incomplete dominance.incomplete dominance. • If an organism shows If an organism shows

INCOMPLETE INCOMPLETE DOMINANCEDOMINANCE for a for a trait, the hybrid will trait, the hybrid will show a show a blendblend of the of the two traits.two traits.

Human Hair Texture

• Because neither curly nor straight hair is recessive, no lower case letters are used.

• CC = curly hair

• CS = wavy hair

• SS = straight hair

These basic laws of inheritance control These basic laws of inheritance control the traits of all living organisms on the traits of all living organisms on

Earth. Earth.