Dominant & Recessive Traits

Chapter 12

DOMINANT TRAITS RECESSIVE TRAITSeye coloring brown eyes grey, green, hazel, blue eyes

vision farsightedness normal visionnormal vision normal visionnormal vision nearsightedness, night blindness,

color blindness*

hair dark hair blonde, light, red hair non-red hair red hair curly hair straight hair full head of hair baldness* widow's peak normal hairline

facial dimples no dimples features unattached earlobes attached earlobes

freckles no freckles broad lips thin lips

Dominant Trait Recessive TraitAppendages extra digits normal number

fused digits normal digits short digits normal digits fingers lack 1 joint normal joints limb dwarfing normal proportion clubbed thumb normal thumb double-jointedness normal joints bent pinky straight pinky

other immunity to poison ivy susceptibility to poison ivy normal pigmented skin albinism normal blood clotting hemophilia* normal hearing congenital deafness normal hearing & speaking deaf mutism normal- no PKU phenylketonuria (PKU)

*sex-linked traits

Longer 2nd toe is dominant over 2nd toe shorter than big toe.

Mendel’s experimentGregor Mendel conducted experiments that used monohybrid crosses; carried out in 3

stepsMonohybrid cross- studies 1 trait

Step 1: Mendel allowed pea plants to self-pollinate for several generations to get

offspring that are true-breeding (purebred). He used true-breeding plants as the first

generation in his experiment. These were called the parental (P) generation

Mendel’s experimentStep 2: Mendel crossed 2 P generation plants

with contrasting traits. This produced offspring called the first filial generation (F1 generation). Mendel recorded the # of F1 generation plants

with each trait.Step 3: Mendel allowed the F1 generation to self-pollinate, producing a second filial (F2) generation. He then recorded the # of F2

generation plants with each trait.

Mendel’s ResultsF1 generation- 100% displayed the same trait

for a given character (the dominant trait)F2 generation- the trait that disappeared in the F1 generation reappeared. The ratio of

the characters in this generation were approximately 3:1 (dominant:recessive).

Notice this # is approximate. Why?

Law of Segregation

When an organism produces gametes, each pair of alleles

is separated and each gamete has an

equal chance of receiving either one

of the alleles.

Patterns in Inheritance?Mendel conducted another experiment to see if traits were passed down in a pattern. He did this by conducting a dihybrid cross.

dihybrid cross- involves 2 charactersHe found that the inheritance of one trait did not affect the inheritance of any other trait. This is because of the Law of Independent

Assortment.

Law of Independent AssortmentDuring gamete

formation (meiosis) the alleles of each

gene segregate independently. Alleles can “mix

and match” i.e. Round seeds can be green or yellow (as can

wrinkled seeds)

Exceptions:

Genes that are located close together on the same chromosome will rarely separate independently. These genes

are linked genes.

Sex Linked Traits•Genes located on one of the sex chromosomes (X or Y)•Most sex linked traits are on the X chromosome because it is much longer than the Y chromosome•More common in males since they only have one X chromosome Examples: colorblindness, hemophelia

More complicated traitsMost traits do not follow Mendel’s pattern of inheritance because most genes either have

more than 2 possible alleles OR are controlled by more than 1 gene.

Examples: Polygenic inheritance

Incomplete dominanceMultiple allelesCodominance

Polygenic InheritanceCharacters that are controlled by more

than one gene are polygenic traits. Examples: eye color (amount of greenness or browness of the eye), height, skin color

Incomplete dominance2 dominant alleles occur. Neither allele is

completely dominant over the other. Example: snapdragons

Multiple Alleles

Genes that have 3+ possible alleles have multiple alleles.

Only 2 alleles for a gene can be present in one individual. The

determination of dominance in these cases can be very complex.

Example: blood type (alleles= IA, IB, i)

Codominance2 traits can appear at the same time for some characters, leading to codominance. In these cases, both alleles for the same gene are fully

expressed.Examples: applies to blood type AB, roan fur color

in cattle

PedigreeA family history that shows how a trait is inherited over

several generations.A pedigree chart is a diagram that shows the occurrence

and appearance or phenotypes of a particular gene or organism and its ancestors from one generation to the

next.A pedigree can help us learn

about sex-linked traits,

dominance, & heterozygosity.

Affects of the environmentSome phenotypes can be affected by conditions

in the environment (such as nutrients & temperature).

Examples: color of the arctic fox is affected by temperature, height in humans can be affected by nutrition, exposure to too much oxygen in

premature babies leads to blindness while too little oxygen leads to brain damage, drugs that cause birth defects (thalidomide prescribed to treat morning sickness in pregnant women),

sunlight & butterfly wing color