Genes and Heredity

Genes and Heredity

Biological traits/allele

Similarities in appearance

Characteristics

Explanation of diversity

What are Traits?

The characteristics an organism possesses. Eye Color Hair Color Height Body Structure Facial Features Skin pigmentation

How are traits passed on?

When organisms reproduce, traits are passed from parent to offspring.

These traits are carried in DNA, the genetic material found in a cell’s nucleus.

DNA acts like a blueprintor the instructions to createyour characteristics

A section of DNA responsible for a specific trait is known as an Allele. An allele is the two or more forms of the gene.

Genes and Heredity

Heredity is the passing of traits from parents to offspring

Genes control biological traits Found on the chromosomes in every cell of the body Characteristics come from a combination of both

parents. Half of your chromosomes come from your mother,

half from your father.

Probability – The odds

It is important to know if observed traits are the result of chance, or other factors. If the environment caused the characteristic, then it is not genetic.

Collect data and see if the results are due to chance.

If a trait is due to genetics then the results will show statistical significance.

***Sample size improves the chances of achieving the expected results and increases our confidence in the results.

Inheritance – Early Beliefs

The idea of inherited traits dates back 6000 years Evidence of this is seen through

stone carvings of pedigrees including horse generations and cross pollination of date palms.

Early records by Chinese farmers show evidence of methods used for improving rice crops.

These ideas were based strictly on observation and trial and error investigation – no isolated experimentation.


Early naturalists believed: many organisms came from cross-

species mating or hybrids Giraffe comes form the mating

of a camel and a leopard

Crossing traits make a blend Tall with short parent will produce mid sized offspring


Many cultures have recognized the value in upholding valued characteristics

Breeding practices for livestock and work dogs. Seed selection and cross pollination of crops Intermarriage of upper class and royalty to

segregate mating of the elite Infanticide of the weak or individuals

who have undesirable traits

Gregor Mendel Heredity meets Science

Considered the father of genetics, Gregor Mendel was born in 1822 in Austria. He grew up on his family’s farm and was able to learn a lot about flowers and fruit trees. After going to college, he joined a monastery. At the monastery, he worked in the garden where he studied how traits were passed from parent to offspring.


Mendel noticed that some patterns of inheritance made sense and other did not. For example, Mendel noticed that when he crossed a purple flowered pea plant with a white flowered pea plant, that all of the offspring had purple flowers. He then noticed that if he crossed two of these offspring, then one out of every four offspring had white flowers. Mendel wanted to know why.


In addition to flower colour, Mendel observed that the garden peas were either green or yellow and were also either round or wrinkled.

He noticed difference in plant heights as all were either tall or short with none in between

There was also variance in flower position on the stem

He felt these traits were significant and began experimentation on them.


Mendel observed seven traits in the peas that seemed to show consistent patterns including seed shape, seed colour, flower colour, pod shape, pod colour, flower position and plant height.

He intentionally fertilized plants by cross pollination by removing the stamen from one plant and transferring the pollen to the pistil of another plant.


Tall plants with short plants made tall offspring. (recall early belief of crossing making a blend – not so!)

Purple flowered plants with white flowered plants made purple flowered offspring.

Round seed with wrinkled seeds made round seeds.

Yellow seeds with green seeds made yellow offspring


Mendel concluded that there must be “factors” that were later identified as genes that are passed on that are more dominant than others.

Dominant genes determine the expression of the genetic trait in offspring

Recessive genes are overruled by dominant genes but are expressed if no dominant gene is present.

Mendel's Laws of Heredity

1. Law of Unit Characters

Inherited characteristics are controlled by factors that occur in pairs. During cross fertilization, each parent contributes one of these factors.

round wrinkled

RR rr

R R r r

Offspring


2. Law of Segregation

The pair of factors separate or segregate during the formation of sex cells

round wrinkled

RR rr

R Rsex cells form r r

Offspring


3. Law of Dominance

One factor can mask the effect of another if it is dominant. The dominant gene is always indicated by a capital letter (R) a lower case letter (r) is used for the recessive trait

round wrinkled

RR rr

R R sex cells form r r

RrOffspringall round


Mendel’s Law of Segregation


4. Law of Independent AssortmentFactors are separated and distributed completely independently of all other factor pairs.

round wrinkled yellow pea green pea

RR rr YY yy

R R sex cells form r r Y Y y y

Offspring Offspring all round (Rr) all yellow (Yy)

A typical Mendelian Cross

Parents tall x short

P TT tt

T T t tFirst generation

F1 Tt Tt Tt Tt

all tall

A typical Mendelian Cross

F1 Cross tall x tall

F1 Tt Tt

T t T tsecond generation

F2 TT Tt Tt tt

¾ Tall ¼ short

Punnett Square

In 1910, Reginald C. Punnett devised a way to organize genetic cross studies.

The Punnett Square is a diagram that is used to predict the probability of an offspring having a particular genotype in a particular cross or breeding experiment.

It is a summary of every possible combination of one maternal allelewith one paternal allele for each gene being studied in the cross.

Punnett Square

Using Mendel’s Crosses

Parents: TT x tt F1cross: Tt x Tt

T T

t Tt Tt

t Tt Tt

Possible gametes

} {Probable offspring

Possible gametes

T T

t

t

T t

T

t

TT Tt

Tt tt

F1 are all tall F2 are ¾ tall ¼ short

Punnett Square

Solve TT x Tt

TT x Tt

Punnett Square

Solve TT x Tt

TT x Tt

T T

T

t

Punnett Square

Solve TT x Tt

TT x Tt

T T

T TT TT

t Tt Tt

All Tall……. But are they the same?

Describing Alleles

Solve TT x Tt

TT x Tt

Phenotype: Description of the trait

expressed. ie. tall or short.

In this case phenotype is 100% tall.

Genotype: The actual gene make up

Representation with letters

In this case genotype is 50% TT and 50% Tt

T T

T TT TT

t Tt Tt

Describing Alleles

The genotype can be described using letters or either of the following terms.

Homozygous - both of the genes are the same and are considered pure – TT or tt

The recessive phenotype always has the homozygous genotype.

Heterozygous - the genes are different and are considered hybrids – Tt

Punnett Square

When solving with a Punnett Square, always use the following steps:

• Define symbols:

• State the cross• Diagram the gametes• Complete the squares• Summarize the results:

and answer the question

GenotypePhenotype

T = tall allelet = short allele

Punnett Square

1. If a short pea plant is crossed with a homozygous tall pea plant, what are the probable offspring?

2. If a heterozygous purple flowered plant is crossed with a white flowered plant, what are the flower colours of the offspring?

3. If two round pea plants are crossed and some of the offspring have round seeds while others are wrinkled, what were the genotypes of the parents?

Punnett Square

1. If a short pea plant is crossed with a homozygous tall pea plant, what are the probable offspring?

T = Tall tt x TTt = short

Phenotype: 100% TallGenotype: 100% Tt (heterozygous)

T T

t Tt Tt

t Tt Tt

Punnett Square

2. If a heterozygous purple flowered plant is crossed with a white flowered plant, what are the flower colours of the offspring? P = Purple Pp x pp

p = white

Phenotype: 50% purple, 50% white

Genotype: 50% Pp (heterozygous)

50% pp (homozygous recessive)

P p

p Pp pp

p Pp pp

Punnett Square

3. If two round pea plants are crossed and some of the offspring have round seeds while others are wrinkled, what were the genotypes of the parents?

R = Round Rr x Rr

r = wrinkled

Phenotype: 75% round, 25% wrinkled The parents must

Genotype: 50% Rr (heterozygous) be heterzygous

25% RR (homozygous dominant) to have both types

25% rr (homozygous recessive) of offspring.

R r

R RR Rr

r Rr rr

Pedigree Charts

A Pedigree is a chart that shows the genetic relationship between individuals in a family

It can be used to determine whether an allele is dominant, recessive, sex-linked or something else.

male without trait male with trait

female without trait female with trait

Unless otherwise stated, squares represent males, circles represent females and shaded markers indicate individuals who have the trait

Pedigree Charts

1. Is the trait dominant or recessive?

2. Fill in the genotypes wherever possible.

The trait must be recessive.

tt

tt tt

tt

tt

Tt

Tt

Tt

Tt

Tt

Tt

TtTt

Tt

T_

T_

T_

Likely TT

Pedigree Charts

1. Is the trait dominant or recessive?

2. Fill in the genotypes wherever possible.

The trait must be dominant.

ee

ee ee

ee eeee

ee

ee

eeEe

Ee

EeEe

Ee

E_

E_ E_ E_

Ee

Documents

Genes and Heredity