HUMAN BIOLOGY

iNtroDUCtioN to GeNetiCs 377

A A a a

b b BB

A A a a

BB b b

B A A B b a b

b b

a

A A a a

B B

A A

BB

b A

bb

A b B a a

a a

B

A Initial chromo-

some alignments

(at metaphase I):

B The resulting

alignments at

metaphase II:

C Possible

combinations

of alleles in

gametes:

One of two possible alignments The only other possible alignment

AB ab Ab aB

Figure 19.6 Animated! In independent assortment, chromosomes and the genes they carry are moved at random into forming gametes.

How do probability and independent

assortment affect inHeritance?

• Probability helps determine a potential child’s chances of
  inheriting a particular genotype and thus for having a particular
  phenotype (trait).


• Independent assortment occurs during meiosis. The individual
  gene alleles on chromosomes are sorted into gametes without
  regard to which other genes the gamete may get.


• Due to independent assortment, the probability is high that
  meiosis will yield gametes—sperm and eggs—having all
  possible combinations of parental genes.

taKe-Home messaGe

rule applies to all the parent chromosomes. Hence meiosis

can produce gametes having all the possible combinations

of parental genes.

Consider how a person may inherit both a chin fissure

and freckles. The genes for these traits each have a dom-

inant and a recessive allele: dominant alleles C for chin

fissure and F for freckles and recessive alleles c and f. If

both parents are heterozygous for both traits, they are both

CcFf. As a result, they can each produce equal proportions

of four types of gametes:

1/4 CF 1/4 Cf 1/4 cF 1/4 cf

The Punnett square in Figure 19.7 shows the probabili-

ties that a child of these parents will inherit a particular

combination of the traits in question. Multiplying four

kinds of sperm times four kinds of eggs tells us that sixteen

different gamete unions are possible when each parent is

heterozygous for the two genes. The key below the Pun-

nett square shows the possible phenotype combinations. It

also lists the probability that any one child will have one

of those combinations, such as a chin fissure and freckles.

Figure 19.7 Animated! Tracking two traits shows the results of
independent assortment. (© Cengage Learning)

1/16

CCff

1/16

Ccff

1/16

Ccff

1/16

ccff

1/16

CCFf

1/16

CcFf

1/16

CcFf

1/16

ccFf

1/16

CCFf

1/16

CcFf

1/16

CcFf

1/16

ccFf

1/16

CCFF

1/16

CcFF

1/16

CcFF

1/16

ccFF

1/4

Cf

1/4

cf

1/4

CF

1/4

cF

1/4

Cf

1/4

cf

1/4

CF

1/4

cF

9/16 or 9 chin fissure, freckles

3/16 or 3 chin fissure, no freckles

3/16 or 3 smooth chin, freckles

1/16 or 1 smooth chin, no freckles

meiosis,

gamete formation

meiosis,

gamete formation

Both parents CcFf

(Possible phenotype

combinations)

1/16

CCff

1/16

Ccff

1/16

Ccff

1/16

ccff

1/16

CCFf

1/16

CcFf

1/16

CcFf

1/16

ccFf

1/16

CCFf

1/16

CcFf

1/16

CcFf

1/16

ccFf

1/16

CCFF

1/16

CcFF

1/16

CcFF

1/16

ccFF

1/4

Cf

1/4

cf

1/4

CF

1/4

cF

1/4

Cf

1/4

cf

1/4

CF

1/4

cF

9/16 or 9 chin fissure, freckles

3/16 or 3 chin fissure, no freckles

3/16 or 3 smooth chin, freckles

1/16 or 1 smooth chin, no freckles

meiosis,

gamete formation

meiosis,

gamete formation

Both parents CcFf

(Possible phenotype

combinations)

parent chromosomes can line up before the pairs separate.
The lining-up is random, so a given chromosome and its
genes may end up in any of eight gametes—and the same

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