How are genes inherited?

The parentsHomer has straight hair Marge has curly hair

The families

Is hair curliness controlled by genes or by the environment?

Homer• Homer has straight hair

• Every cell in Homer has two sets of

genes

• Homer’s has two straight hair genes

• We can show the straight hair gene

with a capital S

• Homer’s genes: SS

Marge• Marge has curly hair

• Every cell in Marge has two sets of

genes

• Marge has two curly hair genes

• We can show the curly hair gene with

a small s

• Marge’s genes: ss

Making a new Simpson• Homer makes sperm

• Each sperm has one gene for hair type

• Homer only has straight hair genes

• So all of his sperm have straight hair

genes

Homer’s Sperm

S S

Making a new Simpson• Marge makes eggs

• Each egg has one gene for hair type

• Marge only has curly hair genes

• So all of her eggs have curly hair

genes

Marge’s Eggss s

Making a new Simpson

s

sS

S

S

S

S S s

s

s

s

Making a new SimpsonSs SsSsSs

• All of the Simpson babies have one gene for curly hair and

one gene for straight hair

• So what type of hair do they have?

Making a new SimpsonSs

THEY ALL HAVE STRAIGHT HAIR!

• The straight hair gene is DOMINANT

• Just one copy of this gene will give you that characteristic

• The curly hair gene is RECESSIVE

• You need two copies of the gene to have that

characteristic

Liking donuts• Homer likes donuts

• Every cell in his body has two donut

genes.

• His genes are: D D

Homer’s sperm

D D

Liking donuts• Marge doesn’t like donuts.

• Every cell in her body has two no

donut genes

• Her genes are: d d

Marge’s Eggs

d d

Making a new Simpson

d

dD

D

D

D

D D d

d

d

d

Making a new SimpsonDd DdDdDd

• Liking donuts is dominant

• Do the Simpson children like donuts?

Weight• Marge has normal weight.

• Both of her weight gens are for normal

genes

• Her genes are: N N

Marge’s Eggs

N N

Weight• Homer does not have normal weight.

He is obese.

• Both of his weight genes are for

obesity (not normal weight)

• His genes are: n n

Homer’s sperm

n n

Making a new SimpsonNn NnNnNn

• Normal weight is dominant

• What weight do all the Simpson children have?

PracticeHomer’s

characteristicHomer’s genes Marge’s

characteristicMarge’s genes

Likes beer BB Doesn’t like beer bb

Plays sports SS Doesn’t play sports ss

Burps BB Doesn’t burp bb

Lazy LL Not lazy ll

Can’t dance dd Can dance DD

No musical ability mm Musical ability MM

The next generation?• Lisa has straight hair

• Every cell in Lisa has two sets of genes

• Lisa has one gene for straight hair,

which is dominant.

• She also has one gene for curly hair,

which is recessive.

• Lisa’s genes: Ss

The next generation?• Milhouse has straight hair

• Every cell in Milhouse has two sets of

genes

• Milhouse has one gene for straight

hair, which is dominant.

• He also has one gene for curly hair,

which is recessive.

• Milhouse’s genes: Ss

The next generation?• Milhouse’s genes: Ss

S s

S s

• Lisa’s genes: Ss

The next generation?

S

ss

S

s

S

s S s

s

S

S

The next generationSS ssSsSs

• Straight hair is dominant

• How many children have straight hair?

• How many have curly hair?

The next generationLisa’s

characteristicLisa’s genes Milhouse’s

characteristicMilhouse’s genes

Likes School Ss Like School Ss

Good eyesight EE Bad eyesight ee

Brave Bb Not brave bb