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MODULE #: 7�POULTRY GENETICS

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THE TRUTH ABOUT CHICKEN

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Chickens have gotten bigger every year – they must be given hormones!

SOURCE: Zuidhof et al., 2014 J. Appl. Poult. Res. 93(12):2970-2982

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https://bodybalance4you.wordpress.com/2013/07/05/non-organic-vs-organic-meat/

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But hormones are NOT used in the production of poultry meat. Aside from the fact that it is illegal to do so, it is not necessary. Chickens have been selected over the years for faster growth.

If a package of poultry meat is labeled as having no added hormones, it must have a disclaimer that says federal regulation prohibits the use of hormones in poultry.

So how did this come about? To distinguish themselves from the beef industry, which can use added hormones, some poultry producers started labeling their poultry as being grown without hormones. But in doing so, people began to assume that if one company is saying they don’t use hormones, the others must be. That was never the case. The practice started to distinguish chicken from beef, not chicken from one company from another. As long as poultry producers continue to market their chicken as grown without hormones, people will continue to be misinformed about poultry production practices.

SUGGESTED ACTIVITY: Social science

A discussion of the power of suggestion and misleading labels

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GENETICS

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Red Jungle Fowl

Meat chicken

(Broiler)

Egg producer

(Layer)

Purebreds

(For show)

Dual purpose

For chickens, genetic selection has occurred over hundreds of years.

All the different breeds of chickens that exist today are descendants from the Red Jungle Fowl of Southeast Asia. After the chicken was domesticated, poultry keepers started selecting birds for different traits. The result has been chicken breeds developed specifically for meat production or for egg production. Just like they have dairy cow and beef cows, there are meat-type chickens, referred to as broilers, and egg-type chickens referred to as layers. There are also less selected dual purpose breeds which are reasonably good at both meat and egg production, but not as good in either as the specially selected breeds.

And just like there are many different breeds of dogs, there are different breeds of chickens that are raised for show.

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The poultry breeder companies use specific breeding programs in the development of different strains of chickens. They have different pure lines of chickens that they maintain. These pure lines have been selected for specific traits such as age at sexual maturity, feed efficiency, egg size, etc. Males from one pure line are crossed with females from another to create the grandparents. Hens and roosters from these hybrid crosses and then bred together to create the parent stock. The parent stock then produce the eggs that are set in incubators to hatch out the chicks that will eventually become layers.

Crossing a rooster and hen from the commercial layer line will not have the same production efficiency as those produced through this selection purpose. As a result, new chicks must be purchased each year rather than breeding your own birds. This is much like the case with hybrid corn and other field crops, where new seed is needed each year.

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CHROMOSOMES

Packaged and organized structure containing most of the DNA of a living organism

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DNA

Deoxyribonucleic acid
A molecule that carries most of the genetic instructions used in the growth, development, functioning and reproduction of all known living organisms and many viruses

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GENES

Basic physical unit of heredity
Linear sequence of nucleotides along a segment of DNA that provides the coded instructions that lead to the expression of hereditary characteristic

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ALLELES

One of two or more alternative forms of a gene and are found at the same place on a chromosome

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BIRDS HAVE MICRO-CHROMOSOMES

Very small chromosomes

CHICKENS

8 pairs of chromosomes (including a pair of sex chromosomes)
32 pairs of micro-chromosomes

HUMANS

23 pairs of chromosomes = 46
No micro-chromosomes

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CHROMOSOMES

Genotype

Genetic make up of an organism

Phenotype

Physical trait observed from the genetics of an organism

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MENDELIAN GENETICS

Homozygous: Alleles are the same on both chromosomes
Heterozygous: Alleles are different on the two chromosomes

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COMB TYPES

SINGLE

ROSE

PEA

CUSHION

V-SHAPED

BUTTERCUP

WALNUT

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GENETICS OF COMB TYPE

Rose = RR

Single = rr

ROSE COMB: Dominant (R)

SINGLE COMB: Has recessive rose comb gene (r)

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		ROSE COMB RR
		R	R
SINGLE COMB rr	r
SINGLE COMB rr	r

PUNNETT

SQUARE

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		ROSE COMB RR
		R	R
SINGLE COMB rr	r	Rr	Rr
SINGLE COMB rr	r	Rr	Rr

PUNNETT

SQUARE

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		ROSE COMB Rr
		R	r
ROSE COMB Rr	R
ROSE COMB Rr	r

PUNNETT

SQUARE

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		ROSE COMB Rr
		R	r
ROSE COMB Rr	R	RR	Rr
ROSE COMB Rr	r	Rr	rr

PUNNETT

SQUARE

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		ROSE COMB Rr
		R	r
ROSE COMB Rr	R	RR Rose	Rr Rose
ROSE COMB Rr	r	Rr Rose	rr Single

PUNNETT

SQUARE

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MENDELIAN GENETICS

Have looked at traits with 2 distinct forms
Not all genes exhibit such simple inheritance

Alleles interact
Genes interact

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GENETICS OF COMB TYPE

Pea = rrPP

Rose = RRpp

Comb type is not that simple though. Comb type is determined by two genes – One for pea comb and one for rose comb.

Chickens with a pea comb have the recessive alleles for the rose comb and at least one of the dominant alleles for pea comb.

On the other hand, the chickens with rose combs have the dominant alleles for rose comb and recessive alleles for the pea comb.

Interesting things happen when you cross a chicken with a pea comb and one with a rose comb.

Rose comb chicken breeds/varieties

Dominiques

Wyandottes

Rose comb variety of Rhode Island Reds

White Dorkings (other varieties have single comb)

Redcaps

Some varieties of Leghorns (also has single comb varieties)

Some varieties of Minorcas (also have single comb varieties)

Some varieties of Anconas (also have single comb varieties)

Hamburgs

Pea comb chicken breeds/varieties – size will vary considerably

Buckeye

Brahma

Cornish

Sumatra

Cubalayas

Aseels

Shamos

Araucanas

Ameraucanas

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POSSIBLE OFFSPRING

PARENTS

Rose = RRpp

Pea = rrPP

Walnut = RrPp

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		PEA COMB rrPP
		rP	rP
ROSE COMB RRpp	Rp	RrPp	RrPp
ROSE COMB RRpp	Rp	RrPp	RrPp

RrPp = Walnut

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POSSIBLE

OFFSPRING

PARENTS

Walnut = rRpP

Walnut = rRPp

Walnut:

RrPp

RRPP

Rose:

Rrpp

RRpp

Pea:

rrPp

rrPP

Single:

rrpp

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		WALNUT COMB RrPp

WALNUT COMB RrPp

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		WALNUT COMB RrPp
		RP	Rp	rP	rp
WALNUT COMB RrPp	RP
	Rp
	rP
	rp

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		WALNUT COMB RrPp
		RP	Rp	rP	rp
WALNUT COMB RrPp	RP	RRPP	RRPp	RrPP	RrPp
	Rp	RRPp	RRpp	RrPp	Rrpp
	rP	RrPP	RrPp	rrPP	rrPp
	rp	RrPP	Rrpp	rrPp	rrpp

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		WALNUT COMB RrPp
		RP	Rp	rP	rp
WALNUT COMB RrPp	RP	RRPP	RRPp	RrPP	RrPp
	Rp	RRPp	RRpp	RrPp	Rrpp
	rP	RrPP	RrPp	rrPP	rrPp
	rp	RrPP	Rrpp	rrPp	rrpp

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		WALNUT COMB RrPp
		RP	Rp	rP	rp
WALNUT COMB RrPp	RP	RRPP	RRPp	RrPP	RrPp
	Rp	RRPp	RRpp	RrPp	Rrpp
	rP	RrPP	RrPp	rrPP	rrPp
	rp	RrPP	Rrpp	rrPp	rrpp

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		WALNUT COMB RrPp
		RP	Rp	rP	rp
WALNUT COMB RrPp	RP	RRPP	RRPp	RrPP	RrPp
	Rp	RRPp	RRpp	RrPp	Rrpp
	rP	RrPP	RrPp	rrPP	rrPp
	rp	RrPP	Rrpp	rrPp	rrpp

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		WALNUT COMB RrPp
		RP	Rp	rP	rp
WALNUT COMB RrPp	RP	RRPP	RRPp	RrPP	RrPp
	Rp	RRPp	RRpp	RrPp	Rrpp
	rP	RrPP	RrPp	rrPP	rrPp
	rp	RrPP	Rrpp	rrPp	rrpp

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COMB TYPES

SINGLE

ROSE

PEA

CUSHION

V-SHAPED

BUTTERCUP

WALNUT

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Buttercup/V-shaped

Duplex comb gene

Comes in 2 forms

V for V-shaped combs
C for Cup-shaped combs

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Breda

Recessive gene

bd – Almost complete lack of comb and wattles
Bd – required for chickens to produce a comb

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CHROMOSOMES

Autosomes versus sex chromosomes

Sex chromosomes deal primarily with sex determination
Autosomes deal with all the other genetically determined traits

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SEX CHROMOSOMES

Mammals: X and Y

XX = females
XY = male

Birds: Z and W

ZZ = male
ZW = female

Your gender was determined by X and Y chromosomes, same as most other creatures from humans to fruit flies.

For humans, the male is XY and the female is XX. Females can only donate an X chromosome to their offspring, while males can donate an X or a Y. Genetically speaking, therefore, it is the male that determines whether their offspring is a male or female.

Birds, butterflies, and snakes, however, take everything we know about sex determination at the chromosomal level and stand it on its head.

Instead of X and Y chromosomes, they have Z and W chromosomes.

What's more, the Z and W chromosomes flip the sex determination system.

A male chicken - or peacock, or giant river prawn, or komodo dragon - has ZZ chromosomes. A female has ZW chromosomes. So a male chicken can donate only Z chromosomes to the offspring while a female can contribute a Z or W. It is the female chicken, therefore, that determines the gender of the offspring.

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EGG INDUSTRY

Problem:

Only females lay eggs
Chicks hatch 50% male

Solution: Sexing of day-old chicks

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SEXING DAY OLD CHICKS

Vent sexing

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MENDELIAN GENETICS

Homozygous: Genes are the same on both chromosomes
Heterozygous: Genes are different on the two chromosomes
Hemizygous: A gene is present on only one chromosome, such as with the sex chromosomes in female birds

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SEX-LINKED TRAITS

Traits only found on Z chromosome

Slow versus fast feathering

Slow dominate over fast

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Z^sZ^s

Z^SW

SLOW FEATHERING

MALE

Z^sW

Z^sZ^S

POSSIBLE OFFSPRING

PARENTS

SLOW FEATHERING

FEMALE

FAST FEATHERING

FEMALE

FAST

FEATHERING

MALE

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FEMALE

MALE

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FEMALE

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FEMALE

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MALE

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FEMALE

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MALE

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MALE

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MALE

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SEX-LINKED TRAITS

Traits only found on Z chromosome

Barring versus non-barring (Barred dominate over non-barring)

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SEX-LINKED TRAITS

NON-BARRED MALE

(Rhode Island Red)

BARRED FEMALE

(Barred Plymouth Rock)

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Z^bZ^b

Non-barred

Male

Z^BW�Barred

Female

BARRED

MALE

NON-BARRED

FEMALE

Z^bW

Z^bZ^B

POSSIBLE OFFSPRING

PARENTS

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Z^bZ^b

Non-barred

Male

Z^BW�Barred

Female

BARRED

MALE

NON-BARRED

FEMALE

Z^bW

Z^bZ^B

POSSIBLE OFFSPRING

PARENTS

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BLACK SEX-LINKED CHICKS AT HATCH

MALES

FEMALES

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SO WHAT?�WHY CARE ABOUT POULTRY GENETICS?

To explain why hormones are NOT needed in poultry production
Sexing day-old chicks
Genetic selection to meet production goals

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DIFFERENT TYPES OF TRAITS

QUANTITATIVE	QUALITATIVE
Traits are measured, rather than counted	Traits are more often qualitative, at best they can be counted
Traits show a continuous variation	Traits are all-or-none, no variability
Traits are controlled by polygenes, each having a minor effect, minor genes	Traits are more often controlled by one or few genes having a major effect, major genes
Expression of the trait is often influenced by the environment	Expression of the trait is independent of the environment
Phenotypic classes do not reflect the genotype classes	Phenotypic classes, on most occasions are good indicators of the genotype

A Mendelian trait is one that is controlled by a single locus in an inheritance pattern. In such cases, a mutation is a single gene is inherited according to Mendel’s laws.

This is called qualitative.

Mendel explained inheritance in terms of discrete factors – genes – that are passed along from generation to generation according to the rules of probability. Mendel’s laws are valid for all sexually reproducing organisms, including garden peas and poultry. However, Mendel’s laws stop short of explaining some patterns of genetic inheritance. For most sexually reproducing organisms, cases where Mendel’s laws can strictly account for the patterns of inheritance are relatively rare. Often, the inheritance patterns are more complex.

These are quantitative traits.

Differences in phenotypes due to genotypes are known as additive genetic variation. Quantitative traits are mostly under the influence of additive genetic variance.

Heritability is the proportion of phenotypic variance due to the additive genetic effects of genes.

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LAYERS		BROILERS
TRAIT	HERITABILITY, %	TRAIT	HERITABILITY, %
Egg shape	60	6 week body weight	45
Egg weight	55	Total feed consumption	70
Adult body weight	55	6 week feed conversion	35
Shell texture	25	Breast fleshing	10
Albumen quality	25	Fat deposition	50
Body depth	25	Dressing, %	45
Age at sexual maturity	25
Keel length	20
Overall egg production	15
Blood spots	15
Adult livability	10
Hatch of fertile	10
Chick livability	5
Fertility	5

If a character is mostly dependent on environment, then heritability is low.

The heritability of fertility, for example, is low at only 5%. Then it is an environmental problem and not a genetic problem.

Growth characteristics are mostly under the influence of additive gene action. Growth characters can be improved through selection. If the heritability of any character is higher then we will make progress through genetic selection.

Correlation is a term which describes how two variables tend to move together. Correlation may be positive or negative. It ranges between -1 to +1 through O.

When two variables move in the same direction the correlation is said to be positive. Sometimes variables move in opposite direction then the relationship between such variables is termed negative correlation. In correlation, we do not consider independence or dependence of any variable.

Knowledge of heritability is needed to formulate effective breeding plans for improvement in next generation.

Knowledge of heritability determines that whether the trait under consideration is controlled by additive gene action or non-additive gene action.

Heritability is also useful for the decision on how much emphasis should be given for selection.

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