Patterns of Inheritance

Mendelian Inheritance

Key concepts

• Mendel’s research showed that traits are inherited as discrete units.

• Mendel’s data revealed patterns of inheritance• The same gene can have many versions• Genes influence the development of traits• The inheritance of traits follow the rules of

probability• A monohybrid cross involves one trait• A dihybrid cross involves two traits.

Case study: Sudden Death on the Court

• http://www.clevelandleader.com/node/6446• Michael Phelps'

• Read Sudden death on the court of Flo HymanHandout to be made from the ninth Edition• Share your understanding of genetic disorder

with your partner

What is the Physical Basis of Inheritance?

Genes are sequences of Nucleotides at specific locations on chromosomes.

• A chromosome consists of a single double helix of DNA made up of nucleotides.

• Genes the units of inheritance are parts of chromosomes.

• A gene’s physical location on a chromosome is called locus.

• The chromosomes of diploid organisms occur in pairs called homologues.

Alleles

• Both members of a pair of homologues carry the same genes, located at the same loci. However, the nucleotide sequences of a given gene may differ in different members of a species, or even on the two homologues of a single individual. These different versions of a gene at a given locus are called alleles.

• The relationships among genes, alleles, and chromosomes can be seen in the next slide.

Each chromosomes carries a different allele of this gene, so the organism is heterozygous at this locus

Both chromosomes carry the same allele of the gene at this locus, and so the organism is homozygous at this locus

Mutations are the source of Alleles

• Alleles arise as mutations that slightly change the nucleotide sequences.

• If a mutations occurs in the cells that become sperm or eggs, it can be passed on from parent to offspring.

• Most of the alleles in an organism’s DNA first appeared as mutations in the reproductive cells of the organism’s ancestors, perhaps millions of years ago, and have been inherited generations after generations.

Homozygous and Heterozygous

• A diploid organism has pairs of homologous chromosomes, and both members of a pair contain the same gene loci and so the organism has two copies of each gene.

• If both homologues have the same allele at a given gene locus, the organism is said to be homozygous at that locus

• If two homologous chromosomes have different alleles at a locus, the organism is heterozygous at that locus.

• The organism that are heterozygous at a specific locus are often called hybrids

Homozygous and Heterozygous

The Secrets of Mendel’s Success

The three key steps to Mendel’s Success• Choosing the right organism with which to work• Designing and performing the experiment correctly• Analyzing the data correctly

Parts of edible pea flower

Useful link to know about Mendel’s Experiment

Edible Pea flower

• Mendel chose the edible pea as the subject for his experiments.

• In pea flowers, the petals enclose all of the reproductive structures, preventing another flower’s pollen from entering.

• Therefore the egg cells in a pea flower must be fertilized by sperm from the pollen of the same flower and this process is called self-fertilization.

• Mendel also opened the pea flower and removed its stamens, preventing self fertilization. Then he dusted the sticky tip of the carpel with pollen from the flower of another plant and this is cross fertilization

Traits of Pea Plants studied by Mendel

Mendel chose to study individual characteristics (called traits) that had different forms such as white versus purple flowers. He started out to study one trait at a time. He followed the inheritance of these traits for several generations, counting the number of offspring with each type of trait.By analyzing these numbers, the basic patterns of inheritance became clear

4.3.1 Define genotype, phenotype, dominant allele, recessive allele, recessive allele, codominant alleles, locus, homozygous,

heterozygous, carrier, and test cross

• Genotype: • Genotype: The symbolic representation of a pair of alleles

possessed by an organism, typically represented by two letters.

• The genetic constitution of an organism• Examples: TT, Tt, tt.Phenotype• Phenotype: The characteristics or trait of an organism- the

way in which the genotype is expressed (appearance of an organism)

• Examples: five fingers on each hand, color blindness.

Definitions

• Dominant allele• An allele that has the same effect on the phenotype whether

it is paired with the same allele or a different one. Dominant alleles are always expressed in the phenotype.

• An allele that has the same effect on the phenotype in a heterozygous individual (where it is combined with a recessive allele) as in a homozygous individual (where there are two copies of the dominant allele)

• Recessive allele: an allele that has an effect on the phenotype in homozygous individuals (where there are two copies of the recessive allele). In heterozygous individuals the recessive allele is hidden by the dominant allele.

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• Homozygous: having two identical alleles of a gene.• Examples: TT is a genotype which is homozygous

dominant whereas tt is the genotype of someone who is homozygous recessive for that trait.

• Heterozygous: Having two different alleles of a gene.• Example: Tt is a heterozygous genotype.• Test cross: testing a suspected heterozygote by

crossing it with a known homozygous recessive.

Definitions

• Codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote.

• Locus: the particular position on homologous chromosome of a gene. Each gene is found at a specific place on a specific pair of chromosome. So alleles of the same gene occupy the same locus.

Monohybrid Cross Animation

• http://www.siskiyous.edu/class/bio1/genetics/monohybrid_v2.html

MonoHybrid Crosses: a cross involving one trait • Experiment: Crossing a homozygous purple flowered plant • with a homozygous white flowered plant

F1 = a hybrid formed by crossing differing pure-breeding parents.

F2 = the offspring produced by inbreeding the F1.

Homozygous = both alleles the same.Hetrozygous = both alleles the different.

Appearance = phenotypeGenetic make-up = genotype

4.3.2 Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnet grid.

Monohybrid Cross• We can study one of the characteristics Gregor Mendel used in his experiments. He studied

the size of pea plants and found that ‘tall’ is dominant over ‘short’. If we start the experiment with 2 pure breeding (homozygous) plants of contrasting traits (tall and short), we will obtain an F1 (First Filial generation) which has the dominant phenotype (tall) but is heterozygous.

• When self-fertilizing the F1, we will obtain an F2 (Second filial generation) which will appear ¾ dominant (tall) and ¼ recessive (short)

• Start by writing the phenotypes for the relevant gene and what the corresponding genotypes are as in the following table.

• Possible phenotypes• Corresponding genotypes• Tall• TT or Tt• Short• Tt• Then write out the cross. Make sure to include both genotypes and phenptypes of the

parents (P), as well as the genotype and phenptype of the offsproing (F1).

Monohybrid Cross

http://www.jdenuno.com/PDFfiles/Monohybrid.pdfYou can go to this site and have practice for Monohybrid cross using Punnet grid.

Drag and drop Genetics Monohybrid

• http://www.zerobio.com/drag_gr11/mono.htm

• Practice Monohybrid cross by using this link

Incomplete Dominance

Incomplete Dominance• When the heterozygous phenotypes is intermediate between the

two homozygous phenotypes, the pattern of inheritance is called incomplete dominance.

• In humans, hair texture is influenced by a gene with two incompletely dominant alleles which we call C1 and C2.

• A person with two copies of the Copies of the C1 allele has curly hair; two copies of C2 allele produce straight hair.

• Heterozygotes, with C1C2 genotype have wavy hair.• Work out the cross between two parents having wavy hair and

show parental, F1, and F2 offspring.

Laws of Mendel

Law of SegregationThe pairs of alleles on homologous chromosomes

separate, or segregate, from each other during meiosis. As a result, each gamete receives only one allele of each pair

Mendel's law of independent assortment, states that allele pairs separate independently during the formation of gametes. This means that traits are transmitted to offspring independently of one another.

• http://www.siskiyous.edu/class/bio1/genetics/dihybrid_v2.html

• Animation on Dihybrid cross

F

1

Phenotypes

RrYy (Selfed)

Genotypes

Round Yellow

Proportions

100%

meiosis

Gametes Y y

R RY Ry

r rY ry

Dihybrid cross genetic diagram

Gametes Y y

R RY Ry

r rY ry

fertilisation

F2 Genotypes RY Ry rY ry

RY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

Dihybrid cross genetic diagram

Dihybrid cross proportions

Phenotypes ProportionsRound Yellow 9/16 or 56.25%

Round Green

Wrinkled Yellow

Wrinkled Green

Dihybrid cross proportions

Phenotypes ProportionsRound Yellow 9/16 or 56.25%

Round Green 3/16 or 18.75%

Wrinkled Yellow

3/16 or 18.75%

Wrinkled Green

1/16 or 6.25%

Dihybrid test cross

• In monohybrid crosses, to know if a dominant trait is homozygous (RR) or heterozygous (Rr) it is necessary to carry out a test cross

• This is done with a homozygous recessive (rr) individual

• The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (rryy)

Dihybrid test cross

Phenotypes

Round Yellow X Wrinkled Green

Genotypes RrYy rryy

Gametes RY, Ry, rY, ry ryGenotypes RY Ry rY ry

ry RrYy Rryy rrYy rryy

Phenotypes

Round Yellow

Round Green

Wrinkled Yellow

Wrinkled Green

Proportions

1/4 or 25%

1/4 or 25%

1/4 or 25%

1/4 or 25%

Dihybrid Cross Problem

• In summer squash, white fruit color (W) is dominant over yellow fruit color (w) and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d).. If a squash plant true-breeding for white, disk-shaped fruit is crossed with a plant true-breeding for yellow, sphere-shaped fruit, what will the phenotypic and genotypic ratios be for:

• a. the F1 generation? b. the F2 generation?

http://www.personal.psu.edu/hw7/Biology110/dihybrid.htmYou can check your answer

Useful links and reference

• http://www.siskiyous.edu/class/bio1/genetics/dihybrid_v2.html

• Animation on Dihybrid cross• http://www.zerobio.com/drag_gr11/mono.ht

m• Drag and drop Genetics Monohybrid• http://www.dnalc.org/resources/animations/• Animations on Genetics•