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MENDELIAN GENETICS. Laws of Heredity. Origins of Genetics. Passing characteristics from parent to offspring is called heredity Accurate study of heredity began with Austrian monk Gregor Mendel at his monastery gardens. Mendel used different varieties of garden pea plant

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Mendelian genetics

MENDELIAN GENETICS

Laws of Heredity


Origins of genetics
Origins of Genetics

  • Passing characteristics from parent to offspring is called heredity

  • Accurate study of heredity began with Austrian monk Gregor Mendel at his monastery gardens


  • Mendel used different varieties of garden pea plant

    • Could predict patterns of heredity which form modern-day genetics principles

    • Garden peas have eight observable characteristics with two distinct traits that Mendel counted and analyzed with each cross or breeding




  • Parent plants are P generation organs (male stamen [pollen] & female pistil) are internal

  • All offspring are F generation (from Latin filialis for son/daughter)

    • F1 generation = first offspring (children)

    • F2 generation = second offspring (grandchildren)

P

P

F1

F1

F1

F2

F2

F2

F2


Monohybrid Cross organs (male stamen [pollen] & female pistil) are internal

P

  • Cross-pollinated two pure-bred plants with one very different trait (purple vs. white flowers) in P generation

    • Examined each F1 plant’s trait & counted them

  • Allowed F1 generation to self-pollinate to produce F2 generation

    • Examined each F2 plant’s trait & counted them

F1

F1

F2

F2


  • Mendel collected tons of data – his results are reproducible

    • Monohybrid cross of white flowers and purple flowers in P generation produced 100% purple flowers in F1 generation

    • Self-pollination of F1 produced 705 purple flowers & 224 white flowers in F2 generation

F1

P

F2


  • Mendel’s ratios still hold true today reproducible

    • Crossing pure bred traits in monohybrid cross will ALWAYS express only one trait in F1

    • Self-pollinating F1 will ALWAYS result in 3:1 ratio

      • 705:224 = 3:1


Heredity theories laws
Heredity Theories & Laws reproducible

  • Mendel knew all ideas about “blending” characteristics was bogus

    • Developed four hypotheses:

      • 1. individual has two copies of gene, one from each parent

From Dad

From Mom


A

b

C

D

e

F

A

B

C

d

e

F

From Dad

From Mom


  • 3. if two different alleles occur together, one may be expressed while other is not – dominant and recessive

    • UPPERCASE alleles are dominant alleles

      • Trait gets expressed ALWAYS

    • lowercase alleles are recessive alleles

      • Trait only gets expressed if dominant is not present

A

B

C

d

e

F

A

b

C

D

e

F

B trait will be expressed

From Dad

From Mom

e trait will be expressed


  • When both alleles are identical, individual is considered expressed while other is not – homozygous for that trait

    • Homozygous dominant = both dominant (UPPERCASE)

    • Homozygous recessive = both recessive (lowercase)

  • When alleles are different, individual is considered heterozygous for that trait

A

B

C

d

e

F

A

b

C

D

e

F


QUIZ YOURSELF expressed while other is not –

TT = ?

Homozygous dominant

Tt = ?

Heterozygous

tt = ?

Homozygous recessive

XX = ?

Homozygous dominant

Homozygous recessive

rr = ?


b

b

P

p

H

H

a

A


General rules for genes
General Rules for Genes for each trait

  • Each gene is given allele letter

    • Letter is always first letter of dominant trait

      • Ex: yellow peas are dominant over green peas

        • Y = yellow, y = green

      • Ex: purple flowers are dominant over white flowers

        • P = purple, p = white

YY

Yy

yy

PP

Pp

pp



  • Some traits are and tall stems are dominant over short stems.dominant – only one dominant allele needed in genome to show phenotype

  • Some traits are recessive – both recessive alleles needed to express phenotype


Polydactyl pp or pp
Polydactyl (PP or Pp) and tall stems are dominant over short stems.


No hitchhiker s thumb t
No Hitchhiker’s Thumb (T) and tall stems are dominant over short stems.

tt

TT or Tt


Tongue rolling r
Tongue Rolling (R) and tall stems are dominant over short stems.

RR or Rr

rr


Free hanging earlobes f
Free-Hanging Earlobes (F) and tall stems are dominant over short stems.

FF or Ff

ff


Widow s peak w
Widow’s Peak (W) and tall stems are dominant over short stems.

WW or Ww

ww


Brown eyes b
Brown Eyes (B) and tall stems are dominant over short stems.

BB or Bb

bb


Left Thumb on Top and tall stems are dominant over short stems.(L)

LL or Ll

ll


Mid digit hair h
Mid-Digit Hair (H) and tall stems are dominant over short stems.

hh

HH or Hh


Cleft chin c
Cleft Chin (C) and tall stems are dominant over short stems.

CC or Cc

cc


Dimples d
Dimples (D) and tall stems are dominant over short stems.

dd

DD or Dd


Freckles f
Freckles (F) and tall stems are dominant over short stems.

FF or Ff

ff


Laws of heredity

t and tall stems are dominant over short stems.

T

Laws of Heredity

  • During meiosis (forming haploid gametes from diploid cells), chromatids separate during anaphase II

    • Law of segregation: two alleles for character separate when gametes are formed

Alleles segregate (separate) into gametes

Female Parent (Tt)

Male Parent

(Tt)

t

T


Alleles segregate (separate) into gametes and tall stems are dominant over short stems.

Alleles pair up in all combos

Alleles segregate (separate) into gametes

Alleles pair up in all combos


  • Mendel studied whether different characteristics were inherited together or separately

    • Conducted dihybrid crosses where two traits are studied

    • Concluded that traits NOT inherited together & developed law

      • Law of independent assortment: alleles of different genes separate independently during gamete formation in meiosis


Law of independent assortment

TB inherited together or separately

Tb

tB

tb

Law of Independent Assortment

Male Parent (TtBb)

Traits separate independently

Female Parent (TtBb)

Tb

TB

tB

tb

TTBB

16 total!

TTBb

TtBb


Punnett square
Punnett inherited together or separately Square

  • Easiest way to represent Laws of Segregation and Independent Assortment is through Punnett Square

    • Cross a homozygous dominant yellow pea with a green (homozygous recessive)

    • Genotypes: YY & yy (1 trait, 4 alleles = 4 combos)

Y

Y

Step 1: separate alleles from genotypes & place on top & down side

Yy

Yy

y

y

Step 2: determine possible combinations by crossing alleles

Yy

Yy


Y inherited together or separately

Y

Yy

Yy

y

  • Have to analyze findings from crossings

    • Genotypic ratio:

    • Phenotypic ratio:

y

Yy

Yy

  • 4 Yy

  • 4 yellow peas


Curi family eye color
Curi inherited together or separately Family Eye Color

  • My dad has green eyes

  • My mom has brown eyes

    • Knowing that I have brown eyes, what is my GENOTYPE?

      • Brown is dominant (B)

      • Green is recessive (b)

        • Dad must be bb

b

b

Bb

B

Bb

B

Bb

Bb


  • According to inherited together or separatelyPunnettSquare, all my parents’ children should have BROWN eyes

    • In reality, my brother has green eyes. What does this mean?

      • Mom’s genotype must be Bb

b

b

B

Bb

Bb

This means that there is a 50% (2/4 or ½) chance that each child my parents had could have green eyes. I lucked out. 

b

bb

bb


Monohybrid cross examples
Monohybrid Cross Examples inherited together or separately

  • 1. Aliens with two eyes are dominant over aliens with one eye. Cross a heterozygous two-eyed male with a homozygous one-eyed female.

    • Genotypic ratio:

    • Phenotypic ratio:

t

t

Tt

Tt

T

t

tt

tt

  • 2:2 (2 Tt: 2 tt)

  • 2:2 (2 two eyes: 2 one eye)


o

O

o

oo

Oo

o

Oo

oo

  • 2:2 (2 Oo: 2 oo)

  • 2:2 (2 orange: 2 purple)


  • Dihybrid (two traits) cross can be trickier male purple carrot with a heterozygous orange carrot.

    • Cross heterozygous purple flowers, heterozygous yellow pea with another of the same.

    • Genotypes: PpYy & PpYy (2 traits, 8 alleles = 16 combos!)

Py

pY

PY

py

Step 1: find possible gametes (two traits each!) for each parent by doing FOIL method (first, outside, inside, last) & place on top & down side

PY

PPYy

PpYY

PpYy

PPYY

Py

PPYy

PPyy

PpYy

Ppyy

pY

PpYY

PpYy

ppYY

ppYy

Step 2: determine possible combinations by crossing alleles, making sure same alleles are together

py

PpYy

Ppyy

ppYy

ppyy


Py male purple carrot with a heterozygous orange carrot.

pY

PY

py

PY

PPYy

PpYY

PpYy

PPYY

Py

PPYy

PPyy

PpYy

Ppyy

  • Analyze results

    • Genotypic ratio:

    • Phenotypic ratio:

pY

PpYY

PpYy

ppYY

ppYy

py

PpYy

Ppyy

ppYy

ppyy

  • 1:2:2:4:1:2:1:2:1 (1 PPYY: 2 PPYy: 2 PpYY: 4 PpYy: 1 PPyy: 2 Ppyy: 1 ppYY: 2 ppYy: 1 ppyy)

  • 9:3:3:1 (9 purple/yellow: 3 purple /green: 3 white/yellow: 1 white: green)


Simple dihybrid rules
Simple male purple carrot with a heterozygous orange carrot.dihybrid rules …

  • Always will be maximum of 4 phenotypes

    • Trait A vs. Trait B, Trait C vs. Trait D = 4 phenotypes

      • AC, AD, BC, BD

  • Heterozygous AaBb vs. Heterozygous AaBb will always have same phenotypic ratio

    • 9 AB, 3aaB, 3Abb, 1aabb = 9:3:3:1

  • Heterozygous AaBb vs. Homozygous aabb will always have same phenotypic ratio

    • 4 AB, 4 aaB, 4 Abb, 4 aabb = 4:4:4:4


Dihybrid cross examples
Dihybrid Cross Examples male purple carrot with a heterozygous orange carrot.

  • 1. Red ants are dominant over black ants, and long antennae are dominant over short antennae. Cross a black short antennae male with a heterozygous red long female.

    • Genotypes:

    • Gametes:

  • rrll & RrLl

  • rl, rl, rl, rl & RL, Rl, rL, rl


RL male purple carrot with a heterozygous orange carrot.

rl

Rl

rL

RrLl

Rrll

rrLl

rrll

rl

  • Genotypic ratio:

  • Phenotypic ratio:

RrLl

rl

Rrll

rrLl

rrll

RrLl

Rrll

rrLl

rrll

rl

RrLl

Rrll

rrLl

rrll

rl

  • 4:4:4:4 (4 RrLl: 4 Rrll: 4 rrLl: 4 rrll)

  • 4:4:4:4 (4 red/long: 4 red/short: 4 black/long: 4 black/short


  • GgSs & GgSs

  • GS, Gs, gS, gs & GS, Gs, gS, gs


gS dominant over no spots. Cross a heterozygous green spotted female with the same type of male.

gs

GS

Gs

GGSS

GGSs

GS

GgSS

GgSs

  • Genotypic ratio:

  • Phenotypic ratio:

GGSs

Gs

GGss

GgSs

Ggss

GgSS

ggSS

gS

GgSs

ggSs

ggss

gs

GgSs

Ggss

ggSs

  • 1:2:2:4:1:2:1:2:1

  • 9:3:3:1


Curi family eye color ear shape
Curi dominant over no spots. Cross a heterozygous green spotted female with the same type of male. Family Eye Color & Ear Shape

  • My father has green eyes and free-hanging ear lobes (homo or hetero?) while my mother has brown eyes (heterozygous) and free-hanging ear lobes (homo or hetero?). What are the possible outcomes for the children?

    • Know that my mother is heterozygous for brown eyes since my brother has green eyes

    • What about free hanging ear lobes?

      • I have free hanging, but my brother and sister are attached! What does that mean about my parents?

        • Both parents MUST be heterozygous for free-hanging ears!


  • bbFf dominant over no spots. Cross a heterozygous green spotted female with the same type of male. & BbFf

  • bF, bf, bF, bf & BF, Bf, bF, bf

BF

Bf

bF

bf

  • Genotypes:

  • Gametes:

BRO

?

?

BbFF

BbFf

bbFF

bbFf

bF

?

SIS

bf

BbFf

Bbff

bbFf

bbff

?

?

bF

BbFF

BbFf

bbFF

bbFf

me

?

bf

BbFf

Bbff

bbFf

bbff


Probability
Probability dominant over no spots. Cross a heterozygous green spotted female with the same type of male.

  • Getting ratios of genotypes & phenotypes is actually calculating probability

    • Probability: likelihood that particular event (genotype or phenotype) will occur

    • Calculated by dividing number of predicted outcomes by number of total outcomes

    • Ex: 3 peas are yellow, 1 is green

      • Words:

      • Ratios:

      • Decimals:

      • Percentages:

      • Fractions:

  • 3 out of 4 are yellow

  • 3:1 yellow

  • 0.75 yellow, 0.25 green (add up to 1.0)

  • 75% yellow, 25% green (add to 100%)

  • ¾ yellow, ¼ green (add to 4/4)


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