Mendelian genetics
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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


Mendelian genetics

  • 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


Pea characteristics traits

Pea Characteristics & Traits


Mendelian genetics

  • In nature, pea plants self pollinate since both reproductive organs (male stamen [pollen] & female pistil) are internal

    • Mendel physically removed stamens & dusted pistils with pollen from chosen plants to observe results by cross-pollination


Mendelian genetics

  • Parent plants are P generation

  • 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


Mendelian genetics

Monohybrid Cross

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


Mendelian genetics

  • 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


Mendelian genetics

  • Mendel’s ratios still hold true today

    • 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

  • 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


Mendelian genetics

  • 2. there exists alternative versions of genes called alleles represented by letters

A

b

C

D

e

F

A

B

C

d

e

F

From Dad

From Mom


Mendelian genetics

  • 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


Mendelian genetics

  • When both alleles are identical, individual is considered 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


Mendelian genetics

QUIZ YOURSELF

TT = ?

Homozygous dominant

Tt = ?

Heterozygous

tt = ?

Homozygous recessive

XX = ?

Homozygous dominant

Homozygous recessive

rr = ?


Mendelian genetics

  • 4. when gametes (sperm/eggs or spores) are formed, alleles for each trait separate independently during meiosis

    • Occurs during anaphase

b

b

P

p

H

H

a

A


General rules for genes

General Rules for Genes

  • 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


Mendelian genetics

  • Ex: In roses, pink petals are dominant over white petals, and tall stems are dominant over short stems.

    • P = pink, p = white

    • T = tall, t = short

      Cross a male heterozygous pink tall with a female homozygous white short

      Male = PpTt

      Female = pptt


Mendelian genetics

  • Some traits are 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)


No hitchhiker s thumb t

No Hitchhiker’s Thumb (T)

tt

TT or Tt


Tongue rolling r

Tongue Rolling (R)

RR or Rr

rr


Free hanging earlobes f

Free-Hanging Earlobes (F)

FF or Ff

ff


Widow s peak w

Widow’s Peak (W)

WW or Ww

ww


Brown eyes b

Brown Eyes (B)

BB or Bb

bb


Mendelian genetics

Left Thumb on Top (L)

LL or Ll

ll


Mid digit hair h

Mid-Digit Hair (H)

hh

HH or Hh


Cleft chin c

Cleft Chin (C)

CC or Cc

cc


Dimples d

Dimples (D)

dd

DD or Dd


Freckles f

Freckles (F)

FF or Ff

ff


Laws of heredity

t

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


Mendelian genetics

Alleles segregate (separate) into gametes

Alleles pair up in all combos

Alleles segregate (separate) into gametes

Alleles pair up in all combos


Mendelian genetics

  • 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

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 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


Mendelian genetics

Y

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 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


Mendelian genetics

  • According to PunnettSquare, 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

  • 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)


Mendelian genetics

  • 2. Orange carrots are dominant over purple carrots. Cross a male purple carrot with a heterozygous orange carrot.

    • Genotypic ratio:

    • Phenotypic ratio:

o

O

o

oo

Oo

o

Oo

oo

  • 2:2 (2 Oo: 2 oo)

  • 2:2(2 orange: 2 purple)


Mendelian genetics

  • Dihybrid (two traits) cross can be trickier

    • 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


Mendelian genetics

Py

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 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

  • 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


Mendelian genetics

RL

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


Mendelian genetics

  • 2. Green frogs are dominant over brown frogs, and spots are dominant over no spots. Cross a heterozygous green spotted female with the same type of male.

    • Genotypes:

    • Gametes:

  • GgSs & GgSs

  • GS, Gs, gS, gs & GS, Gs, gS, gs


Mendelian genetics

gS

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 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!


Mendelian genetics

  • bbFf& 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

  • 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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