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By the end of this section, you will be able to:
  • Explain the relationship between genotypes and phenotypes in dominant and recessive gene systems
  • Develop a Punnett square to calculate the expected proportions of genotypes and phenotypes in a monohybrid cross
  • Explain the purpose and methods of a test cross
  • Identify non-Mendelian inheritance patterns such as incomplete dominance, codominance, recessive lethals, multiple alleles, and sex linkage

The seven characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits. The physical expression of characteristics is accomplished through the expression of genes carried on chromosomes. The genetic makeup of peas consists of two similar or homologous copies of each chromosome, one from each parent. Each pair of homologous chromosomes has the same linear order of genes. In other words, peas are diploid organisms in that they have two copies of each chromosome. The same is true for many other plants and for virtually all animals. Diploid organisms utilize meiosis to produce haploid gametes, which contain one copy of each homologous chromosome that unite at fertilization to create a diploid zygote.

For cases in which a single gene controls a single characteristic, a diploid organism has two genetic copies that may or may not encode the same version of that characteristic. Gene variants that arise by mutation and exist at the same relative locations on homologous chromosomes are called alleles . Mendel examined the inheritance of genes with just two allele forms, but it is common to encounter more than two alleles for any given gene in a natural population.

Phenotypes and genotypes

Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits expressed by an organism are referred to as its phenotype    . An organism’s underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype    . Mendel’s hybridization experiments demonstrate the difference between phenotype and genotype. When true-breeding plants in which one parent had yellow pods and one had green pods were cross-fertilized, all of the F 1 hybrid offspring had yellow pods. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow pods. However, we know that the allele donated by the parent with green pods was not simply lost because it reappeared in some of the F 2 offspring. Therefore, the F 1 plants must have been genotypically different from the parent with yellow pods.

The P 1 plants that Mendel used in his experiments were each homozygous for the trait he was studying. Diploid organisms that are homozygous    at a given gene, or locus, have two identical alleles for that gene on their homologous chromosomes. Mendel’s parental pea plants always bred true because both of the gametes produced carried the same trait. When P 1 plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous    for the contrasting trait, meaning that their genotype reflected that they had different alleles for the gene being examined.

Questions & Answers

diagram of Prokaryotic cells
Magreth Reply
where is it?
Yazi
ʜɪ
Malikie
ʜɪ
Malikie
where is the diagram?
Yazi
waiting.....
Yazi
what
Malikie
good morning guyz
Joelia
gd morning
Hannalyn
morning how are u doing
Paul
doing all fyn en u
Joelia
👍👍👍
Gruxz
morning everyone.. by is Grace we have be saved... Ephesians 2:8....
Cosmo
what is a cell of a bacteria called
Sahfe Reply
what is genotype
James
genotype: this is the combination of alleles an organism has for a given characteristic..
Cosmo
what is maiotic
Gabriel Reply
what is sexual reproduction
Gabriel
what are the effects of concentration gradient in the uptake of water by plant's
Harleen Reply
ls it ok if you have sex during pregnancy
Kags Reply
no
Bernard
yes!!!
Yazi
Yes
Babie
hello
Cabdi
Babie Maseuse .. Hello .. how are you ?
kf
yes it make the baby's strength stay longer
REAL
Who are you?
Babie
why not it's nicer and more enjoyable more than ok for the woman
Lamin
danso from the Gambia and you why do you ask
Lamin
as long as the woman is comfortable and in any possible position good for her
Lamin
unless medically advised to stop or hault and those conditions are also applicable to certain problem in pregancies. Pls any further comment pls never hesitate to ask
Lamin
how tell us
Tony
an assignment for you pls
Lamin
it's recommended
Nyakato
Hello
george
hi how are you?
Lamin
Fine
george
hello everyone
Cosmo
Good night to everyone am from Zambia may the Grace of the Lord be with you all
Cosmo
u too hv gd dreams
Joelia
thanks colleagues and wish you all the best insha Allah
Lamin
InshaAllah
Joelia
Marsha Allah
Lamin
yes
James
What is the meaning of organ
Ronald Reply
tissue combined to form organ
zameer
join
Elishs
orga- these are tissues join together to perform a specific functions.
Elishs
then what are tissues
Kags
group of cells working together to perform a particular function
Harleen
tissue are group of cells put together to perform a certain goal
Bernard
What will happen when read blood cell placed in 0.5 % Nacl solution
zameer Reply
what is biology
Tariro Reply
biology is the study of living organisms
Bernard
examply is the study of.
Nantamu
biology is the study of life.
Nantamu
what is biology
Francis Reply
kidny with large and and many glomeruli are characcteristic feathure of ?
zameer
Can you please put up the notes for antibiotics
Jesus Reply
I can't
Francis
we should first know what is antibiotics.what is antibiotics.
Francis
A cell is the basic functional unit of life.
Jael
what is a cell ?
Rabecca Reply
A cell is the basic functional unit of all organisms
Glandwell
A cell is a basic functional and structural unit of life.
Mavis
please, emphasize more on the coronary circuit
Toyyiba Reply
define the photosynthesis
Sabrinah Reply
list the type of protein
Sabrinah
sugar.glucose
Tamirat
renin,pepsin
Nyakato
peptides
Nyakato
what is photosynthesis
Francis
what is biology
Nathan Reply
branch of science which deal with living and non living things
zameer
different branches of biology
linus
zameer is right
Onuoha
we don't study non living things in biology
mejury
biology is simply the study of life
Toyyiba
biology can simply be the branch of science that deals with the study of flora and fauna
Nyakato
Biology is the study of life
Peace
What is Diffusion
Soliu
The word "Bio" means life and "Logy" means study or knowlegde". Biology is the study of Life (living organisms from microorganisms to plant and animals), their behaviour, mode of living, actions, e.t.c
adewunmi
Bio means life and logy means study of so in other words Biology means study of life
Peace
what is zameer
Francis
what is biology
Pamodou Reply
is the study of life
George
Biology is the study of life
Peace
the word "Bio" is the greek word for "Life" while "Logy" is the knowledge, study of Life. Therefore, Biology is the study or knowledge of life in which case can be from micro organism to plants to animals, their mode of life, features, characteristics, survival e.t.c
adewunmi
biology is the study of living organisms
mejury
study of living organisms
Moses

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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